From what I have read, people can store about 2000-2500 calories of glycogen. When you exercise really hard, you burn pretty much nothing but glycogen. There are various estimates for how many calories one burns per mile but mine is close to 50. So using that, I would suffer glycogen depletion after 40-50 miles of very intense riding. That is assuming, of course, that I had my glycogen stores at max. when I rode.

Now when you ride at lower intensity, you burn a mix of fat and glycogen with increasingly more fat at lower intensities. I think the fat burning may require a certain amount of conditioning. The human body seems hard wired to horde fat as a protection against that famine coming up (which does not come because we live in the land of Big Macs). So I think in people who are fairly out of shape or do not do much aerobic activity, their bodies may just burn glycogen.

Now I have lost weight surprisingly fast cycling. I went on a two week tour once in the Rocky Mountains. We did 70+ miles per day with 4,000+ feet of climbing per day. We also camped so there was quite a bit of activity with lugging ones stuff from the truck and setting up the tent and walking to the showers, and walking to eat, and walking to go to the bathroom and so on.

I had some extra weight on me so I decided to see if I could lose a little weight. I figured that each day's activities would come close to depleting my glycogen so I ate as much complex carbs as I could (fruits, vegetables, starches, even some candy). I did avoid fatty foods and I went light on the meat.

I felt fine the whole trip. I did not have any problems with fatigue or pooping out on climbs. I generally did not push it really hard on climbs because I never knew when they would end and I wanted to keep something in my legs. However, I did get into a feel climbing competitions with other riders on the tour so I did push at times.

I got home and I was amazed to find that I had lost 10 lbs in 14 days and from the change in my waistline it was all fat. So one can lose a lb of fat per day, just watch what you eat and do a ridiculous amount of exercise.

There were people who gained weight on the tour. They not only ate the breakfast, lunch and dinner provided on the tour and they added an extra lunch usually consisting of the largest cheeseburger in town, with a stack of fries, with a huge soft drink followed by a large hot fudge sundae. So even if you do a ridiculous amount of exercise, you can trump it by consuming a ridiculous number of calories.

I think the mistake most people make is not eating enough carbohydrates to replace glycogen stores.

I went to a spa once with my brother though. The diet there had a very low fat content and a very high fiber and carbohydrate contend. I tended to get very sleepy in the afternoon. My brother claimed that we were not getting "enough fat" and combated this by going out and procuring and devouring a small greasy hamburger in the afternoon. He might have been right.