I have cut-n-pasted the following from the Cyclingnews Fitness Q&A section. Our better weather is coming along and with it I hope you will be able to pile on some miles. Feeling good on the bike is all about combining fitness and fuel. If you don't eat right (or the more common problem - enough) it won't matter how fit you are after about 90 minutes or riding.
Carry enough food that you can begin eating about 45 minutes in and have a big bite every 15-20 minutes throughout the event or session. A total of 250-350 calories per hour works for most athletes but you'll have to find your own level. More is better so long as you don't end up feeling full or bloated.