There's lots of good advice out there about balanced diets - you should try to eat that.
20 miles is not especially far (once you get used to it that is). For something like that I'd eat a bagel and maybe a banana for breakfast. I used to do almost 20 miles to get to work a few years back and quite often I'd eat nothing - but I'd be pretty hungry in the afternoon and wary of the ride home though.
It's natural to feel hungry later. Just think about it - you have a small breakfast, then go out and spend a bunch of calories (probably more than you eat for breakfast).
Today I did 72 miles - with a bunch of hills on the route. I take 2 bagels (about 250 calories each according to the package) to bed with me (with some butter on them). I set my alarm 1.5 to 2 hours before the ride starts and I eat the bagels - and maybe have some water because we get dehydrated overnight. Later I'll have a bowl or two of cereal (a dense cereal) and sometimes a banana (but not today because I don't have any).
During the ride I have a mix in my water bottles (e-Load) that provides some calories and electrolyte replacement - and I bring energy bars (today I had two - Cliff Bars). We ended up having a rather long lunch stop so I ate a panini and some soup.
The best thing (for rides over 2 hours) is to start replacing the energy you use on the ride. I say 2 hours because if you fuel up properly the night before (and eat something for breakfast) your body can store 2 hours worth of useful energy. Your body can also digest about 250 calories per hour, so when I'm on a long ride that's what I try to give it. Cliff Bars are conveniently about 250 calories.
You still have to be careful what you eat. Just because you did a ride doesn't mean you can go crazy and eat anything you want.
