Muscle glycogen stores are depleted in 45-60mi so if you don't begin to eat something
on a next day ride at the 20-30mi mark your legs are going to go flat. You can train to
the point where you can restore muscle glycogen in 24 hrs to full levels and to where
you can metabolize fat and protein more efficiently which is where a good bit of the
energy is going to come from on rides of 100+mi. Attention to hydration is equally
important.