I used to do 200 mile races -- and, for those, I paid a lot of attention to my food/hydration strategy. Food strategy, at that distance, is the difference between success and failure. Everything I ate and drank was very easily digestable and based on the principles found in Alan Lim's Skratch "Feed Zone Portables" book.
I ride centuries non-stop and "at speed," so this has an influence on my food strategy. The night before is a pasta dinner (not extreme -- just enough to make sure my glycogen is topped up) with rice pudding for dessert. The morning of, I have the same breakfast I have every day -- a bowl of cereal. Then, in the hour leading to the start, I drink a bottle of full strength EFS drink mix. The EFS contains enough electrolytes to cause me to retain a LOT of water -- so my body, in effect, holds the contents of an extra water bottle at the start. I also down a Snickers bar in the last 30 minutes before the start.
During the century, I carry a few rice cakes (Feed Zone Portables recipe) and/or some Colombian Bocadillo Guayaba in my jersey pocket. In both cases, they contain a lot of moisture, a lot of calories per bite, and go down (and digest) easily. (Gels don't agree with me and Clif bars don't digest fast enough.) A person can only absorb so many calories per hour, so I base my consumption on time rather than distance. The faster I plan to ride, the less I have to carry.