Walking, or any other cardio excercise along with diet, is the only way to trim that belly. Sit ups DO NOT trim the belly. All it does is strengthen the abs underneath the fat. Kind of like picking up icing on top of your boots (bridgeing). Personally, I think it's about 80/20 diet to excercise that produces a flat belly. Now, when you do lose the belly fat, those crunches will come in handy at that point.