You get an hour for lunch. Beware going out; you don't want to lose focus, get lost, return late, etc.
Eat whatever you enjoy, just eat small amounts of it regularly - before you start, at the morning break, at lunch, at the afternoon break. I chose foods I really liked, so each break was like a reward for getting to that point. I stayed wide awake from 6 a.m. to 6 p.m.
I would've thrown up if I'd eaten eggs for breakfast. Protein shakes, pieces of fruit and hard cheese, bagel, salad, energy bars, dried sweetened pineapple, and limited amounts of water worked for me.
I also brought non-MCAT things to read and CDs to listen to, so I could get my mind off the horribly difficult VR section just before lunch.