Many a time I have seen people carrying around huge coolers with enough food for the entire cast of the magic kingdom!
personally, I wouldnt bother. You can eat pretty cheaply counter service in the parks, and if you are clever you can get some good interesting stuff (not just burger and fries) - or why not eat a GOOD breakfast prior to entering the park, and either eat a counter service meal later on in the day or have a snack in the park, and eat offsite on the way home? We often do that - as we never need two meals a day at Disney. breakfast, snack and dinner is usually plenty for us!
Sammie x