Beat the Mile 20 Wall: Marathon Fueling & Gut Training
Hitting the wall isn't a mystery. It's math. Here's the fueling math, and a prompt that builds your gut training plan.
I always figured hitting "the wall" at mile 20 was some deep physiological mystery. It turns out it's simple math.
The math
Your body stores carbs as glycogen: about 400g in your muscles and 100g in your liver. At 4 calories per gram, that's 2,000 calories in the tank. That's all you get.
Why can't you store more? Every gram of glycogen holds 3 grams of water. Double your range and you'd be carrying another 4.5 lbs. No thanks.
Running costs about 0.7 calories per pound, per mile. At 170 lbs, that's 119 calories a mile, so a marathon costs around 3,100.
2,000 in the tank, 3,100 to spend. You come up 1,100 short, somewhere around mile 17.
That's 275g of carbs you need to take in. Over a 4-hour marathon, it works out to 70g an hour.
Training your gut
If you're like me, learning to fuel hasn't been easy. Plenty of attempts ended in some unpleasant bathroom stops.
So I built a Claude prompt for fueling and gut training. It runs your actual numbers, then builds a gut training progression from whatever you're taking today up to your target, timed to your race. It ends with a race-day fueling strategy you can screenshot.
I put the actual papers into the prompt too, so it works straight off the research.
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