It's a great acheivement. Something all serious cyclists feel they have to do.
I'm doing my second soon. I cycle on weekend with my neighbours. Our century 'training' is to ride 50km (30 mile) every Sunday morning. If you can do 50km easily (if you can still do all your chores the rest of the day), then try 100-120km about a couple weeks before you plan the century. This longer ride will determine not only if you have the stamina for a century, but also if your butt can stand the miles. Its a good time to fine tune your saddle position, or buy new shorts.
As a final prep, don't do a long ride within a week before your ride. Get lots of sleep every night in the week before your ride.