Fats are readily oxidized and contain about twice the energy as carbohydrate or protein (9 calories versus 4 calories per gram). You carry far more metabolizable energy as fat than any other fuel. So, why do runners, bikers, hikers, and, well you name it, consume so much carbohydrate? They have their reasons, but I am not so sure they are that sound.
Consuming high carb diets makes one a very poor mobilizer of fat. This is one path through which high carb diets, in excess of energy expenditure, contribute to the accumulation of fat through a decrease in the rate of fat utilization. So, an endurance athlete who is not good at mobilizing fat as energy is in more danger of depleting muscle glycogen. It is also true that carbs are a potent source of free radicals that cause fatigue and tissue inflammation.
Now, it turns out that women are gaining on men in the ultra marathons that are run over several days in Death Valley (fitting in my mind). One reason given is that they are better mobilizers of fat than men. Using fat as an energy lowers their oxidative load and they do not tire so readily. Women probably eat more antioxidant-rich foods than men, though that is not reported.
Another factor is that being low on antioxidants seems to make one a poor mobilizer of fats as a source of energy. A study in Lipids, 2005; 40(4):433-5 suggests that athletes on a low antioxidant diet (low in vitamin C and beta-carotene) had lower circulating levels of omega-3 and omega-6 fatty acids compared to athletes on the same diet but high in antioxidants.
From an evolutionary perspective, fat was the prefered endurance fuel. The sorts of carbohydrate-laden foods and drinks modern athletes consume did not exist; in fact, there were no simple carbs 100,000 years ago. Our ancestors ate wild plants that were very high in antioxidants and consumed as much fat as they could, meaning that the lean animals they ate had to be processed to capture all their fat content.