http://www.geocities.com/ne_century_rides/training.html

(I googled "training century")

This has a nice chart -- but mainly, just RIDE :-) what's the standard formula - don't increase by too much at once, like 15%... but that depends on your constitution, too.

And if you can't ride as many miles as you feel you should, do the ones you can do faster (or tuck in a few sprints halfway through... makes the whole rest of the ride a little more challenging )