I've done the STP a number of times myself (3 times in 2 days, 2 times in one day), an plan to do it again in 2005. The site recommended mileage building up to the big day is a good start, but I looked for other organized rides to help build up the mileage. Start early though, its easier to build up slowly than to make up for lost time.

The first time I did the one-day ride, I just about killed myself. I didn't do the recommended mileage, invited a gal I was beginning to date to join me at the hotel the night before the ride (big mistake, didn't get enough sleep (if you know what I mean)), then got on a paceline I had no business being on, and didn't drink enough water. I bonked at about 115 miles and it was slow going the rest of the way!

Second time (older and wiser), I came closer to the recommended training mileage, carb-loaded appropriately (had been doing lo-carb through out training), drank plenty of water, wasn't tempted by the pacelines speeding by, and stopped more often to drink/eat/stretch. Only side-effect I had was I lost some feeling in my left hand for a couple of weeks (improper riding position I presume).

If you've never done STP, stick with doing the 2-day ride. It's fun, and even though they'll give you a map to follow, you'll never pull it out as there are always bikes ahead/behind to keep you on the right path.
Good luck