There are two theories but first, a quick physiology lesson. The body burns both fat and glycogen at all times for energy. Fat stores are abundant; glycogen stores are not, but they are the body's preferred fuel.
Got it? Now, here are the schools of thought:
Theory #1 When you run slower than your aerobic threshold roughly the pace at which you can carry on a conversation you burn a higher percentage of fat, sparing glycogen.
"The more often you do that, the more often you will burn more fat on a regular basis and at a faster pace," says Benji Durden. In other words, long, slow running teaches the body to burn more fat.
The wall a feeling of lightheadedness and overall fatigue that hits around 20 miles is the body's reaction to running desperately low on glycogen. But a body that has learned to burn fat manages to "spare" glycogen, leaving more for those crucial miles from 20 to 26.
Theory #2 The body does not learn anything. "The body does not get a baccalaureate degree in fat-burning," says David Martin, a marathon physiology expert. "Glycogen is always burned more readily than fat."
What happens during long runs is that the body runs low on glycogen. Afterward, the body stores an additional amount of glycogen to replenish what it has lost.
"You do this enough times, and rest before your big race, and there's much more stored glycogen than before," Martin says. "Come race day, you can run through the wall."