I was told by a fitness nut case that the stomach area is the hardest part of the body to firm up. Body fat just wants to go right to the stomach area and hang around there. So don't over do yourself and expect it to just disappear. It takes alot of work from my understanding. My uncle has a stomach that sticks out past his belt but instead of it being flabb its hard as a rock. I have never seen anything like it before and don't want to ask about it because he tries to lose weight... so its a touchy subject.
I don't have back problems (that I am aware of) and it kills me to do sit ups. Its like one of my ribs is jamming it into my lung. Kind of hard to describe but I cant do sit ups .... no way.