This is my experience. I did 2 rides this week that were close in length and elevation gain - some big hills (28-30 miles). The first one (28) I was riding with someone who was faster than me and I pushed myself. I also stopped less and ate less. I was really tired when I got home. The second ride (30) was with a friend who rides about my speed. Stopped and had a drink and a few bites and a short lunch break. Got home, felt great and could have ridden longer - no fatigue at all.

I have also discovered that you don't need to eat alot while you are riding, but you need to eat something. If I don't eat when I am riding, I start to feel really not good - it is probably what others have called glycogen depletion. And once that sets in, it doesn't go away right after you eat something. So, I was probably eating too much to compensate for this and sabotaging weight loss. Now, I realize that I can eat 4 pretzels or half an energy bar and avoid glycogen depletion without adding a whole lot of extra calories.