I did my first century last year, and the prep for it was a weekly ride from 45-65 miles for about 7 weeks before the ride, plus the addition of regular 20-30 mile rides I was doing. Never actually did a century until I "did the century".
With a group, some training rides under your belt, and plenty of chamois butter application and food breaks, a century is easily doable, but you do need to get some miles under your belt before hand if you don't want to kill yourself on the day. With the training, I found the Century enjoyable and relaxing, rather than feeling I was going to die.