I would add that one of the harder things about long rides, including centuries, is knowing what and when to eat and drink. and being discliplined about it. Especially in the heat, you have to be pretty discliplined in staying up on your hydration and nutritional needs. Beyond that, it's a bit of a mind game. Your energy may wain here and there and you have to be prepared to work through it.

From my own (limited) experience, a decent mileage base that includes a few rides in the 70-80 mile range, along with hill work if the century is a hilly one, will put you in a good position to do the century comfortably. I did my first century this May at the Horsey Hundred in Kentuckywith a good base and some climbing miles and did just fine IMO. I rode with someone who had a good aerobic base and some climbing in her legs, but less of a mileage base. She did fine too, but the last 20 miles hurt a lot.

By the way, I think it also helps to do long rides with a friend or group. It certainly makes it more fun.