A general guideline - but know thyself, of course! - is that you can go three times as far on a supported, "big day" race as you can ride comfortably a few times a week. Some people do a longer ride once a week to gradually build up closer to the big one - depends more on your schedule, prob'ly. Since I commute and then do club rides, I've got enough "regular" miles to do a century when I want to - of course, knowing how to pace yourself, and staying fed and watered, is also important. IMO it's just as important to have a comfy bike :-) And remember, for every uphill, you get to coast...