In addition to the good training advice you've been give above, be very careful to take in sufficient fluids and food during the century rides. It's especially important that you take in enough salt to replace what you'll sweat out, and enough food that you don't bonk. Even if the ride organizers are providing rest stops, I highly recommend bringing 2 or 3 gels. I normally throw down a PowerGel just before the start of a big climb - the combination of quick carbs and caffeine gives me a physical and psychological boost that helps get me up the hill.
Also, use your heart rate monitor, and don't start out too fast. It's very tempting to latch on to a 24 mph paceline early in the ride, but if there's a bunch of climbing ahead, you might very well regret it.
Good luck!