I think the meal has to be fully digested & converted "fuel" before any of it would get burned by exercise.

some info I found on various exercise websites -

Most sites I read -
suggest waiting 4 hrs to exercise after a big meal, 2 hrs for a moderate meal and so on.

Basically the body burns the stored glucose in the liver & muscles and then will begin to burn the stored body fat after the other sources are depleted.

generally it is better to eat after workouts as the body's metabolism is revved up and will stay revved for awhile after.