You’ve probably heard it before: “No carbs? Say goodbye to endurance.”
Yeah, I’ve been there too—side-eyed by pasta lovers who think you’ll collapse halfway through a 10K.
But here’s the deal: you can train for a marathon on keto. It just takes a different kind of mindset (and maybe more salt than you’ve ever eaten in your life).
Think about it: your body carries enough fat to fuel days of running. Seriously.
One guide points out that fat stores pack about 20 times more energy than glycogen does.
So if you’re fully fat-adapted, that’s like having an ultra tank of fuel.
You might never hit “the wall” in the classic carb-burnout sense.
But let me say this loud and clear—the magic word is adapted. This doesn’t happen overnight.
And get this: even if you’re not keto, your body already tips into ketosis after long runs.
Cleveland Clinic nutritionists explain that after extended effort, you start burning fat more naturally.
So in a weird way, your training is already nudging you toward ketosis.
Still sounds nuts? I get it.
I went keto myself right after a half marathon here in Bali—scorching heat, off-season, no races on the calendar.
It was the perfect time for slow base miles and messy energy shifts.
If you try this two weeks before a race, you’ll feel like you’re dragging sandbags with your legs.
Becoming fat-adapted takes time. Experts say it can take anywhere from 4 to 12 weeks.
I gave myself a solid 8 to 12 before expecting anything close to “normal” on my long runs.
Bottom line? Keto running is a slow burn. You don’t flip a switch and feel amazing.
At first, you’ll feel sluggish. Maybe moody. Maybe “I hate everything” levels of hangry.
Totally normal.
Cleveland Clinic even warns that you won’t feel or perform at your best during this adjustment period.
So put your ego in time-out. Run by effort, not pace, and trust that the steady energy will come.
Mindset Shift: You’re Training Differently Now
Let’s be real—this isn’t just a diet tweak.
Keto messes with everything: hunger cues, energy dips, recovery. Even your easy pace might feel hard.
It’s all part of the deal.
I noticed I was way hungrier at first. Fat digests slower, so it took time for my brain to feel “full.”
Sleep got weird too. And mentally? It felt like I was trying to solve a math test mid-run.
If you’re going this route, you’ve got to let go of the “calories in, calories out” mindset.
That model doesn’t fit here. Instead, focus on how you feel over time.
Those early miles might suck—but if you stay the course, they’ll start to feel better again. Trust me.
Here are a few mindset shifts I leaned on—and still coach others through:
- Be patient – Don’t expect beast mode by week two. It took me about 4 to 6 weeks before my heart rate and pace finally started syncing again.
- Control what you can – Focus on showing up, getting rest, and sticking to the plan. Ignore your pace for now. Heart rate or effort is your real scoreboard. It’s normal to be 10–20% slower during adaptation.
- Support the slump – Early keto feels like you’re dragging a sandbag mentally. That’s okay. Nap more, meditate, take extra rest days. Cleveland Clinic says your body needs time to adjust. Believe them.
- Celebrate the small wins – Ran 5 miles even though your legs felt like stone? That’s a win. Fewer walk breaks? Big deal. These mini-victories add up.
Keto running is humbling.
If you’re the type who chases fast splits, trust me—they’ll come back. Just not right away.
Once you get adapted, it’s like unlocking a new gear you didn’t know existed.
Timing Your Keto Transition for Maximum Success
Timing is everything.
If you flip the keto switch right before a race, you’re setting yourself up for misery. Your body won’t be ready, your brain will rebel, and every run will feel like a death march.
Instead, pick a quiet stretch in your training. Here are some good windows:
- Off-season or base-building – That post-race lull is gold. That’s when I went keto—easy miles, zero pressure.
- Post-goal recovery – You’ve hit your goal, so now’s the perfect time to tinker without tanking performance.
- Injury break or burnout window – Already on a forced rest? Use that time to dial in your new fuel system.
Starting keto mid-cycle—especially during a speed phase or taper—is like swapping engines mid-flight. Don’t do it.
Fat adaptation can take 8+ weeks. I treated my keto shift like a base-building phase.
Lots of slow miles. No chasing pace. I even walked more than usual. That kept my ego in check and my energy steady.
Hydrate, Salt, and Supplement Smart
This part’s not optional.
If you skimp on salt, you’ll bonk hard—and it won’t be because of lack of carbs.
On keto, your body flushes water and sodium like crazy.
Low carbs = low insulin = kidneys dumping fluid.
Result? Headaches. Cramping. Feeling like you’ve been hit by a bus.
So yeah—eat more salt than you think you need. I’m talking bouillon cubes, salt tablets, salty bone broth.
I aimed for 1–2 grams of sodium above normal every day just to stay afloat.
Hydration tip:
Water isn’t enough. You need electrolytes, too.
Plain water can dilute your sodium and make things worse.
Add an electrolyte mix or even just a pinch of salt to your bottle.
Magnesium & Potassium:
Don’t skip these.
Avocados, spinach, nuts, and salmon are all keto-friendly potassium sources.
At night, I’d take 200–400mg magnesium—helps with sleep and stops those twitchy legs.
Most keto flu symptoms (cramps, insomnia, fatigue) are just electrolyte crashes.
Pre-run hack:
Add a spoon of MCT oil to your coffee.
MCTs are fast-absorbing fats that convert quickly into ketones.
One endurance blogger said it kicks in “as fast as sugar, in about 3 steps”.
It gave me a steady kick, especially on early morning runs. Just don’t test anything new on race day.
Pro tip: If your run feels like garbage in week one or two, it might not be fitness—it’s probably salt.
Stay ahead of hydration and don’t wait until you feel thirsty or crampy.
Keep sipping and salting all day long.
Phase 1: The Keto Transition Weeks
Let’s not sugarcoat it—those first two weeks of going keto suck.
You’re running on empty, quite literally. Your carbs are gone, but your fat-burning engine isn’t firing yet.
It feels like your body’s trying to run a marathon on airplane mode.
You’ll probably deal with brain fog, dead legs, and the kind of fatigue that makes you question why you ever started this.
But that’s all part of the switch.
You’re not broken—you’re just rewiring.
Here’s how I got through it (and how I’ve coached runners through it too):
- Back Off the Gas
This is not the time for track sessions or all-out intervals. Your system’s in reboot mode. Push too hard now and you’ll hit the wall—fast. - Stay in Cruise Mode
Zone 1–2 runs only. Easy, conversational pace. And yes, walking is fine. My first 8-miler during this phase was a mess—I had to walk every mile.
I swallowed my pride because I knew slowing down was part of the process. - Add More Rest
If you feel wrecked, skip the run. Do some light biking, swimming, or even a yoga flow. Let your body catch up to your new fuel source. - Dodge the “Keto Flu”
That awful foggy, nauseous feeling? It’s not just carb withdrawal.
Most of the time, it’s dehydration or low electrolytes. Load up on salt, sip on broth, and hydrate like it’s your job. - Plan Around the Chaos
Make your training schedule keto-friendly. For me, I stuck to short, easy runs and added in some chill cross-training—biking, hiking, that kind of stuff.
I cut all speedwork and capped long runs to 4–6 miles.
That way, if I needed to bail mid-run, it didn’t derail my entire week.
Sample 2-Week Transition Plan:
Week 1:
3–4 short easy runs (20–30 mins), one “long” run (~4–5 miles), and 2 full rest or cross-training days.
Absolutely no intensity.
Week 2:
Build the long run to 6–8 miles, 3 runs around 30–45 mins, and 2 recovery days.
Still no speedwork.
During this stretch, your usual pace will feel like molasses.
Don’t fight it.
Focus on effort or heart rate zones, not the numbers on your watch.
The speed will come back—but only if you don’t rush the process.
Pacing Expectations: You’re Not Slower Forever
Here’s the deal: once you’re out of the fog, you’ll still feel slower than pre-keto. That’s normal.
Your body is learning a new way to fuel—switching from quick-burning carbs to long-lasting fat.
Most low-carb runners report being 10–20% slower at first on tempo and even easy runs.
But don’t panic.
Instead of chasing pace, track your heart rate or how you feel.
If you used to hit 9:00/mile at a 150 bpm heart rate and now it’s 10:30/mile at the same HR, that’s not failure—it’s adjustment.
When I was deep in this transition, I watched my easy pace drop from 11:00 to 9:30/mile at the same heart rate over about six to eight weeks.
That was the proof. The work was paying off.
Here’s how to train smart during this phase:
- Use your heart rate, not your ego:
Pace doesn’t matter right now. Comfort and aerobic load do.
Over time, you’ll notice your heart rate getting lower for the same pace—proof you’re becoming more fat-efficient. - Ease intensity back in:
Don’t go from zero to VO2 max. Start small.
Shorter reps, longer recovery, maybe a banana or gel before the session (targeted keto).
That little carb bump can help you hit the workout, then slip right back into fat-burn mode. - Skip the PR chase:
You’re building a new base.
Speed will return—but only once your fat-adapted engine is fully built.
One review of endurance research found keto didn’t give runners a fast performance boost—but given time, keto runners caught up to their high-carb counterparts.
Bottom line: At first, your splits will slow. Then, they’ll bounce back.
If you trust the process, your endurance will grow stronger than ever—and when it’s time to race again, your speed will surprise you.
Advanced Tweaks: TKD & CKD for the Win
You don’t have to stay 100% hardcore keto every second of the day.
A lot of runners—including pros—tweak things a bit for key workouts or long races.
Targeted Keto (TKD)
This is what I call the “banana-before-the-beatdown” method.
You stay low-carb most of the time, but take 20–30g of fast carbs 30–60 minutes before a hard effort—like track work or a long hill grind.
Think small: half a banana, a honey stick, maybe a gel. You’ll burn through it fast, and it won’t kick you out of ketosis long-term.
Cyclical Keto (CKD)
This one’s more for long training cycles or race week.
Go low-carb 5–6 days, then have a carb refeed for 1–2 days to refill your glycogen tanks—like a Friday night carb-up before a Sunday long run.
Big-name ultrarunners do this too.
Zach Bitter uses targeted carbs during races.
Jeff Browning rotates carbs into heavy training weeks.
The key? You’re fueling with intention—not using keto as an excuse to binge on Snickers.
One runner even told me his mid-run “gel” was MCT oil and almond butter. That’s hardcore, but it worked for him.
Figure out what your body responds to, but always come back to this:
Fat is your primary fuel. Carbs are just backup.
What to Eat: Real-World Keto Fueling for Runners
Keto running means ditching the sugar gels and figuring out real-food solutions that won’t spike you out of ketosis mid-run.
Before Runs
Eggs. Bacon. Avocados. Nut butter.
Or my go-to: black coffee with MCT oil (a.k.a. bulletproof coffee).
That combo gives you steady energy without the crash.
During Runs
No candy or syrupy gels here.
Use solid fats—nut butter pouches, cheese cubes, boiled eggs (bonus if wrapped in bacon), or even salty avocado halves.
One runner I know swears by homemade almond butter + MCT + sea salt gel pouches—and crushed a 30K on them.
After Runs
Focus on protein, veggies, and healthy fats.
Think: salmon and spinach, chicken and broccoli, or a giant salad drowned in olive oil.
If you need something sweet, toss in some low-carb berries or make a chia pudding with protein powder and coconut milk.
As one keto-marathoner said: “Eat a lot of vegetables and healthy fats: avocados, nut butters, chia seeds…”
You’re not just fueling—you’re recovering smart.
What to skip?
Pop-Tarts, candy bars, sports drinks.
They’ll not only knock you out of ketosis—they’ll wreck your gut mid-run.
Sample Keto Marathon Training Week (After You’re Fat-Adapted)
Once you’ve been keto for 8+ weeks and the fog’s lifted, your plan might look like this:
Monday:
Full rest or light cross-training (yoga, walking). Keep carbs super low.
If you’re dragging, maybe add a few extra greens or a slice of fruit.
Tuesday:
Easy Zone 1 run (45–60 mins). Just black coffee or tea beforehand. No mid-run fuel needed.
Afterward: eggs + greens + salt.
Wednesday:
Workout day. For speed, take a small carb (like a banana) 30 mins before.
If not, do a fartlek. Keep the total run around 45 mins.
Recover with broth and protein.
Thursday:
Easy run plus strength. Zone 2 only (30–45 mins).
Recover with nuts or cheese.
Friday:
Rest or shakeout jog (20–30 mins, super chill).
If you’re doing CKD, make Friday night your carb meal—maybe rice or sweet potato with dinner.
Saturday:
Long run day (90 mins to 3+ hours).
Fuel with MCT oil, nut butter, salty snacks.
Stay on top of electrolytes.
Refuel with a protein-loaded salad.
Sunday:
Easy run or cross-train (30–60 mins).
If using CKD, today is your carb boost—fruit, sweet potatoes, rice.
If not, just go high-fat with maybe a dessert of berries and cream.
Your weekly mileage, effort, and volume will look similar to your old plan.
The difference?
You’re fueling it with fat—and that changes everything.
Red Flags, Burnout & When to Pull Back
Let’s get one thing straight—keto’s not some miracle shortcut.
It works for a lot of runners, but if things feel off, don’t ignore the signs.
Here’s what to watch for:
- You’re always tired – Like, really tired. If it’s been 12+ weeks and even your easy runs wipe you out, something’s wrong. Most likely? You’re not eating enough—either calories or carbs.
- Mood swings or garbage sleep – If you’re snapping at people or lying awake at night with a racing brain, under-fueling might be the culprit.
And for women, missed periods are a big red flag. - Training has flatlined—or gotten worse – If your pace or stamina has been stuck in a rut for months, that’s your body waving a white flag.
It might be time to try a TKD (targeted keto), CKD (cyclical keto), or just eat more. - Your bloodwork’s off – Keto can mess with cholesterol and thyroid for some folks. Get labs checked. Don’t guess—talk to your doctor.
- Mentally fried? – Look, keto takes willpower. If you’re not just sick of the diet but feel like throwing your entire food routine in the trash, it’s time to reset.
Here’s the deal: keto isn’t one-size-fits-all, and it’s okay to tweak the plan.
Personally, I’ve gone through cycles.
When recovery tanked and I felt wrecked after long runs, I added a few carbs before key workouts.
Some runners do a mini carb load (100–150g) in the 48 hours before a race, and it helps them feel sharp without breaking fat adaptation.
I’ve coached folks who go strict during base training, then loosen up closer to race day.
It’s all about staying healthy, not being dogmatic.
And listen—there’s zero shame in bending the rules.
I had a buddy who ate pizza every Friday night.
Sunday long runs? Total punishment. His body was screaming for balance.
Sometimes “pizza night” is smarter than dragging your butt through a bonk run.
Female Runners: Hormones Matter
Now if you’re a woman training on keto, things can get even trickier.
Very low-carb eating can mess with hormones—leptin, thyroid, cortisol—especially if you’re not fueling enough.
So don’t ignore these signs:
- Track your cycle & energy – If you notice a big energy dip mid-cycle or right before your period, try adding more carbs that week—maybe some fruit or sweet potatoes. It can make a huge difference.
- Eat enough—period – Women tend to be more sensitive to low calories.
Keto isn’t code for “undereat forever.” If your goal is to run stronger, not disappear, you’ve got to eat. - Talk to someone who gets it – If your cycle disappears (amenorrhea), or you just feel off, see a nutritionist who understands female athletes.
You may need to go more cyclic. - Nutrients count too – Don’t just load up on bacon and butter.
You need leafy greens, nuts, seeds, and quality protein to cover your micronutrients—especially iron.
Some women thrive on strict keto. Others? They need a little wiggle room to keep hormones steady.
There’s no gold medal for sticking to 20 grams of carbs no matter what.
Be smart. If fatigue or hormonal chaos creeps in, loosen the grip a little.
I promise, your long runs will thank you.
Gear & Recovery Hacks for Keto Runners
Let’s talk real-world stuff.
Running on keto means you’ve got to prep differently—because no, you can’t just slam a gel every 45 minutes and hope for the best.
Bring your own fuel
Candy’s out.
I carry a tiny flask of MCT oil on long runs (yep, I sip it like it’s whiskey).
Salted nut packs, cheese cubes, even homemade coconut-oil gels work. Some folks toss salt tabs in their belt too.
Tweak your hydration setup
I ditched sugary sports drinks years ago.
Now I run with a belt that holds plain water and a homemade electrolyte mix—just salt and potassium.
On ultra days, I’ve even used bone broth in a flask. Salty and weird, but it works.
Rethink recovery
Post-run, I usually grab protein with some healthy fat—eggs, beef, avocado.
If the run was long, I might throw in a bit of fruit or beet juice to help refuel.
Research shows ketones help with recovery too—boosting glycogen and reducing muscle breakdown.
But don’t rely on fancy science alone—real food still wins.
Watch your salt and thirst
On keto, some folks lose salt fast.
If you’re constantly cramping or your sweat tastes like the ocean, double down on salt intake.
FAQ – Real Talk on Running Keto
Can I run a fast marathon on keto?
Maybe. Don’t expect a magic PR.
A study of 132 endurance runners showed keto didn’t make anyone faster compared to carb-fueled runners.
What keto can give you is more stable energy and easier fat loss.
But let’s be honest—you’re not shaving an hour off your marathon just because you swapped toast for eggs.
Want to boost race performance? Consider a small carb bump before the race.
Will I bonk on keto?
Once you’re fully fat-adapted? Probably not in the same way.
Fat stores are massive, so you’re less likely to crash from glycogen depletion.
That said, the first few weeks of keto feel brutal. You’ll swear you’re bonking every run.
Push through that phase, fuel with fats and electrolytes, and things level out.
Do I need to carb-load before race day?
Not always. Some keto runners skip it entirely and still race great.
Others eat 100–150g of carbs 1–2 days before, just to top off glycogen stores.
I suggest testing it before a big training run first—see how your body responds.
Don’t wing it on race day.
What’s a good keto-friendly energy gel?
DIY wins here.
I’ve seen runners use nut butter packets, hard cheese, boiled eggs, even coconut oil mixed with salt in a squeeze pouch.
There are keto gels out there—usually made with MCT or glycerol—but honestly, real food is cheaper and works great.
The “best” gel is the one that doesn’t wreck your stomach.
Final Thoughts: It’s a Long Game, Not a Shortcut
Training for a marathon on keto isn’t for the faint-hearted.
Your first few weeks will suck. You’ll second-guess the whole thing.
But if you stay consistent and fuel smart, things start to click.
Even the research says it: sticking it out through the rough patches builds serious mental and physical resilience.
I’ve had runs where I felt unstoppable—cruising along without a single carb in my system.
Waking up the next day without soreness? That’s a huge win.
Just give it time. Be patient. Tinker with your plan.
The goal isn’t keto perfection—it’s to run stronger and recover better.
Now it’s your turn:
Have you tried keto for marathon training?
What tricks helped you push through the rough weeks?
Drop a comment. Let’s share wins, failures, snacks—whatever helped you get through it.
Run your race, your way. And don’t be afraid to adapt along the way.