Rule of thumb is to do a long ride on Sat that's about 10% longer than last week.
So you want to be at 60 a couple weeks before (don't forget to take a few days off before the century). So it's 54/50/45/40 etc.. You will get some big jumps in mileage along the way, but you will prob also need a week or two where you just repeat the mileage.

Expect to deal with several problems that you will need to find solutions for. You will need good shorts and chamois creme as well as a bike and a helmet.

There is no substitute for time on the bike. Get rolling, and as soon as you can.
See if there isn't a spinning class you can take.