Adrian, good deal man. A century is doable in that time frame assuming you pace yourself, eat right and it isn't some crazy ride with 10,000 vertical feet in the 100 miles. Just start at reasonable distances, which given your fitness level is probably in the 15 mile range. You'll probably gain strength pretty quickly and soon 40 to 50 miles won't feel like all that much. After that 100 is just pushing yourself a bit harder. Most organized centuries have 4 to 5 stops built in which makes them all the more doable. I wouldn't recommend an unsupported century unless you're riding with a group of friends.
Boo, so psyched for you I can't stand it. I imagine you're bouncing off the walls like a kid on Christmas eve.
Me - barring the third week in a row of bat invasions on a Wednesday night (did I just curse myself of what) I'll be hitting the one big hill around here tomorrow after work with a good friend and fellow Team Big Guy member. Should be a nice, but hot ride. Temps are supposed to creep up on 90...