For long training rides 4-6 hours I usually stick with a couple cups of oatmeal, 2 pieces of toast or Pop Tarts, and a glass (12 oz) of OJ. I may even throw in a glass of slimfast. 

During long training, races, or 24 hour events I always had the old secret nutrition of my grandmothers banana bread.

Scott

What a huge topic this is and important. 

What to eat while on the bike needs to be complimented by what to drink while on the bike!  First the level of exertion is key. This generally means is the ride time greater than or less than 2 hours.  Now a days an electrolyte replacement drink is key and ingesting carbs in the proper ratio to protein helps to refuel you while riding and keep you going faster longer. 

That is the short answer for now.

Check out Edmund R. Burke, Phd's book on  recovery.   Optimal Muscle Performance and Recovery. This book is exellent, period.