Originally Posted by
adamrice
This is all very rule-of-thumb-ish, but my understanding is that, depending on intensity, you can get up to ½ of your calories on a ride from stored fat, and you've got enough readily accessible energy on top of that to get you through about 2½ hours of riding.
I never ride fasted (at least, not intentionally). My approach has been that for rides of less than two hours, I don't bother eating during the ride; for rides of 2+ hours, I have a Clif bar (250 cal) at the end of each hour. I'm probably burning about 600 cal/hour on those longer rides, so—even if I am getting 300 cal/hour from fat—I'm not quite calorie neutral, but so far I've been getting through pretty long rides feeling pretty good at the end, and am not wrecked the next day. I do plan to increase my rate of calorie intake as I increase my ride length.
I don't have a scale at home, so I can't report on weight.
Be aware that the Clif bar you eat now, is for energy that you'll want/need about 1.5 hours after eating it, since those bars are primarily made from complex carbs that take about 90 minutes to be digested. So if you only have 90 minutes or less til the end of your ride, then a Clif bar is not the right choice, and a liquid Gu/Powerbar gel pack is the better option since those are typically made from simple carbs (aka sugar) that break down much faster.