Improving metabolic efficiency
The problem is burning too much carbs/glycogen and not enough fat, since you body has an essentially unlimited supply of the latter.
How? One step is to eat more fat and less carbs. That trains your body to burn more fat. This article also suggests pushing your limits on how far you can go without eating, as a way to train your body to need less. Another point is that you burn more carbs (vs. fat) at higher levels of effort. Do enough training at low effort, and your body will get used to using fat. That's the idea behind Phil Maffetone's training philosophy.