Besides training for stamina and performance, as you increase distance you need to look at eating and drinking habits. This is what makes the biggest difference when you move up from shorter rides. You'll need to get into a habit of consuming throughout the ride during those longer rides (not necessarily for the shorter tempo or hill rides).
Also take a look at the good advice in this thread, if you haven't already:
http://www.bikeforums.net/showthread...ding-a-Century