Originally Posted by Carbonfiberboy
Agree with Machka. Very, very important to start eating at your maximum sustainable rate right from the start. I start eating after 15 minutes.(...)
I think this is a mix of preaching to the choir and some potentially misleading advice.
No one here disputes the usefulness of eating on centuries and ultras. During the double century mentioned above, I ended up finishing nearly an hour longer than I should have, simply because I unintentionally averaged only 120 kcal/hour over the second half. (While we're on this subject, it's also important to drink even if you don't feel like drinking. The reason is that you need water for the carbs to be absorbed. If you go much lower than 24 oz/hour, it inhibits your ability to digest carbs and you may even get nauseous.)
The question is what to do when your legs give out 40 miles into a ride. Here I say that you need to do more riding without on-the-bike carbs because you have insufficient glycogen stores and you need to work on that. Machka mentioned 2000 kcal in glycogen stores. For a trained cyclist who's not doing a time trial, 2000 kcal would easily last 60+ miles. You can count on 200 kcal/hour (conservatively) coming from fat, which, at 15 mph average, provides 800 out of 2000 kcal needed to go 60 miles.
The downside is that you will be less capable of sprinting towards the end of a 40/60 mile ride if you don't eat during the ride.
For bonus points, do what one of my linked studies did, and try some high intensity intervals after 40 miles on water.