i sometimes eat a piece of fruit or bagel before my 12+ mile ride in. always drink at least a full glass of water. sometimes i eat nothing and have no issues.

20 miles is starting to get to the distance where you might wanna have something in the tank before starting. not saying you are gonna crash at mile 19, probably not. i once went on a little joy ride that ended up being thirty miles. i didn't eat before leaving as i didn't expect it to be that long and i crashed...hard at about 26-27 miles. i was seriously out of gas and regretting not eating. the fairly big hill right before my house was no fun that day.