When we are very fit, our glycogen stores can fuel a six-hour hard ride in the mountains, similar to the one my friend took. But after the ride, the glycogen tank is almost empty. In comparison, even a rail thin triathlete stores enough fat to fuel five Ironmans in a row.
Fat is an almost unlimited resource, but it comes with two problems: The human brain is a sugar lover, and the rate at which fat is burned for fuel is too slow to support a hard, fast Ironman effort. In other words, your body fuels itself with a combination of glycogen and fat (and a little protein), with fat being the source of fuel that lasts but which cannot be tapped quickly enough to keep you moving fast.