Well, if I were doing a century, my average speed would be about 13-14 mph so I would be on the bike for 7 or 8 hours. Back in 1983 I did 140 miles in 8:25, but that was then and this is now.

I think the key is nutrition. Never let yourself get hungry. If you bonk because of lack of fuel, you will never be able to really recover during the ride. The damage will have been done.
I know a lot of people ride fueled by gels and sports drinks, but I prefer real food: peanut butter and banana sandwiches, trail mix, fruit, and real water. However, if you run marathons you know what your body needs.