I think the people who are saying "just do it, no training necessary" forget how hard their first century was.
Also, a century isn't as bad as it may seem. I found that running a half marathon was significantly more challenging than a typical century.
The three main reasons to train are, as others have alluded to already:
- avoid overuse injuries
- make sure your bike is set up properly
- make sure you know how to eat and drink enough
Sapling's links look pretty good to me. Make sure your bike fits you properly, and that you get in the habit of consuming about 250 calories and 500-750ml of water or sports drink every hour.