Why do runners hit the wall? What does this mean for the aerobic system?
Runners usually hit the wall because they don’t have well developed aerobic systems, and they can't optimally use fat as fuel. Think of it this way: a marathon is like a long road trip in a car: you want to be in 5 th gear most of the time. If you’re forced to use a lower gear because 5th gear is broken, you’ll use up all your fuel, blow the engine, or both.
The problem is that most runners never develop 5 th gear—their aerobic system (and fat-burning ability)—and so their body is forced to run the race in a much lower gear—one that forces them to burn more sugar. As a result, these runners use up their sugar stores (liver glycogen), which quickly becomes depleted causing a significant loss of energy.
In a properly trained endurance athlete, liver glycogen is used differently: instead of being shunted to the muscles to be burned as the main fuel source, it is used to help the muscles burn fat.
Think of fat as the logs in the fire (a slow, long-term energy burn), and sugar as the kindling (quick, short term energy that helps fat burn). So, even in a highly-trained athlete, fat-burning will always be accompanied by some sugar burning. In essence, greater fat-burning capability will always mean more endurance.