I've been reading Nancy Clark's books on sports nutrition and nutrition for cyclists. Fuel for the ride comes from glycogen, and the amount of stored glycogen you have available in muscles, liver and blood is less than 2000 calories (that may be too high if you weigh less than 150). I don't have the exact numbers, but it was something like 1300 calories worth of glycogen in the muscles and 350 in the liver. After that, you've depleted your energy source.

For me, 2 hours of riding uses about 1200 calories, and I am down to 113 pounds, so I probably have less glycogen available. If I do rides less than 1 1/2 hours, I don't need a snack. Now I know why I really need to start eating if I ride more than 25-30 miles, because I have used up all the available energy. Hope that's helpful