I don't know how all the people run with so much food in their system. Man, I have to keep it at the bare minimum otherwise I pay for it dearly.
I've always run with very little on my stomach, especially any work out that's going to require real effort. To go out for an easy long run is one thing, but to do a harder tempo or interval workout, no freakin' way. I'd be puking or cramping.
You can get used to running on a full stomach. I run ultras, so a 50-mile flat-out effort on hilly trails with a full stomach is just another day at the office.