Sarah's advice on gradually adding milage and using the longer rides to experiment with nutrition/hydration etc is right on.

In preparing for the long ride I did last month, I used one of the many century workout schedules that are on the web. I found it useful as a guide to help keep me on track and not overdue the mileage build up. Between life and work, I couldn't follow it exactly, but modified it to fit my schedule and didn't get too worried when I wasn't staying exactly on track. If I missed rides, I just picked up where I left off. This link shows one chart and has downloadable Word documents for a few other variations.