I bring my lunch and breakfast every day on my backpack. I'm just careful to use stuff that won't spoil. We have a café at work so I don't have to worry about taking milk, but it's really not too hard if you had to bring individual cartons. Just package it in a plastic bag to eliminate leaks.

I like tuna. In a can or in a bag, it lasts a long time.

If you like to turn your tuna into salad, you can get a couple extra mayonnaise packets next time you go to your favorite fast food restaurant (I use Chick-Fil-A), or try this place for single serving packages of just about everything.

http://www.minimus.biz/

This place is great for backpacking supplies...I found out about it in a motorcycle touring magazine. My favorite for clever packaging is the twin pack of PB&J!