Veronica's right, and I'd put them on and do road training just to be used to them by the event- they handle differently.
What you want to do is some really long slow rides for basic conditionig and endurance. Speed just picks up on it's own as you do these. Work on your hamstrings at the gym. Very Tedious! but worth it. Crunchies for core strength!
Actually, you should have been weight training this winter, 'cause now it's time to ride! Swimming is a really good in season crosstraing thing.
If you look around at sites like Bicycling Mag and such, you can get a training schedule for the century that's often modified for the shorter distances.

You'll do great! Even if SAG has to scrape you off the road it'll be fun.

Lizzy