Are you trying to lose weight or maintain where you are now?  I think you have some good habits established by making healthy choices, but I think you should focus on breakfast.  I think oatmeal is a good choice but maybe you should add some protein.  Maybe a hard-boiled egg or whole wheat toast with peanut butter.

Are you drinking any calories (sweet tea, soda, juices, etc)?  If so, you should really look at that and see what makes sense.  Sugary soda pretty much doesn't make any sense to me.  Same for sweet tea (although it's probably a better choice than soda).  Many juices are loaded with calories as well.  Drink plenty of water.  I carry a camelbak better bottle and sip from it throughout the day.  I probably drink 4 24oz bottles of water throughout the day.

Keep a journal of everything you eat throughout the day for a couple weeks.  Be honest and total up the calories as well as the balance of carbs, proteins, and fats and the time you eat throughout the day.  You can then calculate what you take in, and it looks like you have an idea what you burn and that should give you a range of how much you should eat.  Keep focused on eating the right foods and I can't see how you could go wrong.  I also try to balance my intake throughout the day.  300 or so calories for breakfast and about 200 every three hours or so for the rest of the day.

As for having some belly fat, depending on your age it can be very hard to get rid of.  Doing sit-ups will strengthen the muscles under the layer of fat, but you still need to get in your cardio training to lose the fat.  There are calculators online you can use to determine what heart rate you want to target based on your goals...fat-loss, heart health, endurance training, etc.

If you're not doing and kind of strength training already, you should start doing it.  Building strength in your core is a great place to focus, but don't forget about your legs...keeping them strong will help you avoid running related injuries.