More Miles =More Pounds: A Marathon Mystery
How many calories are being burned here?
Last week, I was sweating buckets in the humidity (AKA doing the weekly long run) with my pacing group and the person I was running alongside, we’ll call him Charlie, said:
“I got on the scale last week and I put on three pounds. Isn’t that weird?”
I should mention that “Charlie,” said this with zero judgment, just genuine curiosity. He might as easily have been saying, “Check out that squirrel that swiped a full bagel, I wonder if he can finish it?”
As someone who has spent more years of my life than I care to admit locked in a death stare with the numbers on the scale, I found his reaction impressive. Kind of like people who truthfully say they’re thinking about nothing. It’s low-key enlightenment.
Because from a purely logical, calories-in, calories-out perspective, the idea that you could add MULTIPLE HOURS of running to your weekly schedule and still gain weight feels like a cruel joke from the universe.
Are you there, Universe? It’s me, Molly. I’m running 30 miles a week. Why are my jeans tighter?
Let’s discuss.
Many Runners Gain Weight During Training
Ask a veteran marathoner, or do an internet search, and you’ll see: weight gain during marathon training is common. In most cases, the factors leading to weight gain are common too.
Let’s start with the physiological factors.
Glycogen storage
Glycogen is your body’s storage form of glucose. Glucose is what your muscles love to burn during aerobic exercise. When you tick up the mileage, your body gets the message loud and clear that you’re gonna want to have more glycogen on hand. And here’s where the additional pounds come in: when glycogen is stored in your body, it’s bound to water. By storing more glycogen, you’re also storing more water.
More glycogen = more water weight.
Body repair
Running causes inflammation. Now, before you freak out because “inflammation” is the dirtiest of dirty words in the health sphere these days – know that running workouts cause acute inflammation, not chronic inflammation. Chronic inflammation has been linked to all sorts of icky healthy outcomes, but acute inflammation is, often, just your body’s normal response to healing. After a long run or tough workout, your muscles and joints might become inflamed as they repair normal damage from the stress of running. Inflammation leads to fluid retention.
Body repair working hard = fluid retention = more weight.
Increased Muscle Mass
This one’s pretty straightforward. As a running powerhouse, you’re literally building stronger quads and glutes. (Turn up the beast mode if you’re also including weightlifting in your training plan.)
Muscles grow = more weight. Boom.
This is all good stuff, right? You’re storing glycogen for big workouts; repairing your tissues after those big workouts; and increasing your muscle mass afterwards. While these factors can increase your scale weight, you might also find that your clothes are fitting the same (or better!) In this case there’s probably not much to do differently besides thank your body for being a BEAST of an adaptive machine.
Hunger Games: Marathon Edition
This next set of factors is a bit trickier, because they revolve around hunger—and that means your brain is now fully in the mix. It also means the weight gain you’re experiencing might not just be added muscle or water weight. It could be driven by how and when you’re eating (or not eating) around your runs.
Fuel Timing
After a long run—or any hard cardio session—your muscles are basically screaming:
“You really made me work for two whole hours without food? Feed me already!”
Meanwhile, your stomach is whispering:
“You just shook me like a snow globe for 120 minutes. Kindly leave me alone.”
This mismatch is where a lot of runners miss a key recovery opportunity.
The 30–60 minute window post-run is your body’s prime time for replenishment. Your muscles are like hungry sponges, ready to soak up nutrients, especially carbs to refill glycogen stores and protein to begin repair. (I’m partial to something cold and quick, like chocolate milk or an iced mocha with added protein powder.)
Skip that window and you’re inviting a full-blown T-Rex raid of the kitchen later in the day, because your body is doing everything it can to get what it needs. When your digestive system chats with your muscles and gets back online, it’s like:
“Cool. We waited patiently. NOW HAND OVER ALL THE CARBS BEFORE SOMEONE GETS HURT.”
Fuel Amount
Here’s the thing: most humans—runners included—overestimate calories burned and underestimate calories eaten. Not out of ignorance, but because many of our favorite foods are calorie-dense and without tracking every bite we take (no thanks), it’s hard to keep track.
Let’s take my 2024 marathon as an example. According to my Garmin, I burned 2,472 calories. That’s a big number. A number that absolutely felt like, in addition to my medal, I also deserved a very large milkshake.
Here’s roughly what I ate on that day:
On-course nutrition: ~500 calories
Recovery chocolate milk: 200 calories
Post-race beer and chips at the Race Day Resort: 500 calories
Burger, fries, and another beer for dinner: 1,800 calories
Total: 3,000 calories, give or take.
(For the record: I did zero food tracking on race day. I enjoyed every bite of that burger and every sip of that beer with pure satisfaction.)
This isn’t a cautionary tale about “eating too much.” It’s a reality check about how easy it is to replace calories with delicious, well-earned food after a marathon. That calorie math can be wildly unbalanced if you’re fueling like this regularly during training.
In a kinder universe, running 10, 15, or even 20 miles would automatically come with an unlimited food pass. In this universe, if your goal is to maintain weight, you’ll want to put intentional recovery and mindful fueling into play.
What to Do Instead of Worrying About the Weight
If you’re like “Charlie,” and a couple of new pounds just make you curious—great. Now you know why that scale might nudge up even as your mileage climbs.
But if you feel like you’re getting caught in the exhausting eat–run–weigh–repeat cycle, let’s zoom out. Obsessing over numbers never got anyone to the finish line. These quick tips can help you stay nourished, energized, and sane during training:
Refuel within 30–60 minutes post-run (even if your stomach isn’t quite on board yet)
That post-run window is golden for repairing muscle, replenishing glycogen, and keeping your blood sugar from crashing into hangry dinosaur territory. It doesn’t have to be complicated. My go-to? 12 oz of ice-cold chocolate milk + a scoop of whey protein isolate. It’s quick, carby, and protein-rich—just enough to hold me over until I’m ready for real food.
Eat enough throughout the day (and before you’re full-on starving)
Under-eating early usually leads to overeating late—and not the satisfying kind. I follow up my recovery shake with a solid brunch/lunch that includes protein, carbs, and something green. Bonus points for planning ahead, so you’re not stuck choosing between cold hot dogs and a straight jar of peanut butter when the hunger hits.
Refuel with the good stuff
If your recovery meal includes a big, cheesy slice of deep-dish pizza (preferably from Lou Malnati’s, because you have taste), I fully support that. Just remember: real recovery happens on a cellular level, which means your body also needs proteins, fiber, and nutrients from plants. So throw a salad or some veggies next to that slice. Your mitochondria will thank you.
Ready to Ditch the Weight-Watching Spiral and Fuel Like a Runner?
If you’re putting in the miles but feeling stuck—tired, cranky, weirdly hungry at night, or just confused by the scale—I see you.
And more importantly: I can help.
As a nutrition coach who works with runners, I’ll help you:
Build a fueling plan that matches your body and training demands
Understand your hunger—and head it off before it hijacks your pantry
Embrace real nourishment (without overthinking every bite)
Recover faster, run stronger, and feel amazing
Click here to learn more about working with me one-on-one.
Or, if you’re just dipping a toe in, join my email list. You’ll get weekly real-talk tips on whole-food fueling, training mindset, and how to eat like an athlete—minus the guilt, guesswork, or green juice pressure.
You’re running a marathon.
You deserve to feel amazing doing it.
Let’s fuel that finish line feeling—together.