I'm going to be a bit of a contrarian here -- for one hour of exercise (bike ride, long walk, workout at the gym) I don't find it necessary to eat during the hour.

However I make sure I have eaten appropriately beforehand, such as a snack of about 200 calories 1-2 hours before starting if it's been a while since my last meal. I also make sure not to eat anything too heavy or hard to digest, so nothing too high in fat or with extreme fiber content.

I drink water throughout the exercise.

However I usually have a pack of gu or two in my bike bag, just in case I get hungry on any ride.