I used to do the same thing. My first century I at a ton, and was fairly slow.
This last year, I started using a good energy drink (accelerade for me), and about all I ate during a century was:
1.5 Clif bars
56 oz Accelerade
70(ish) oz water
For me, about 250-350 calories per hour seems to be the good zone - any more and my stomach feels funny. I try to get nearly all of it as carbs, as you have adequate fat reserves to ride essentially forever, as long as you have carbs.
The first year I did the century in 6:49, riding by myself. Last year, on the same route in a group, we did it in 5:35.