Despite all the comments about them, MRE's (Meals ready to eat) are good sources of high energy, and you can get ones that actually taste good. (Ham and the chow mein are actually GOOD). They cost about $7/ea at a camping supply store, and each one is about 3000 calories if you eat everything. They are also lightweight.

Drink LOTS of water, and supplement the water with powdered Gatorade mix that you can get in packets.