Lets start with a decently fat belly .. not the real big one. I say this because you must have enough weight to challenge you a bit, too much of it will make you not want to move and that will lead to morbid obesity. Those who have just enough weight will have to really be fit enough to challenge themselves as opposed to those who are fit they really don't have to be fit to carry themselves. Therefore those who seem fighting fit with just adequate muscles are really not fit in true sense of the word.
So how do you make yourself fit? Firstly you must be happy and always want to have a hearty laugh and you must have no problems in consuming food. Nay I will not say you need to be a foodie or a lover of food, far from it. When you want to eat you must eat and make sure you dont hog alone you need to eat and drink with friends and good company.
Most of all you must have good rest, you need to sleep very well.
With all this make sure you don't worry about your weight or that pot belly of yours. But you need to worry if you are not able to move about or even if you hesitate to move about. Make sure your always ready to walk around run around, climb and trek inspite of your pot belly. Only then you are fit. If not you are just sloppy.