If you cannot get a road bike well before your century (to get comfy and all), then get slicks for your mountain bike and have a blast!
Don't borrow a road bike for the century. Ride the bike you are comfortable on and that is set up for you. If you have the time and money to get a road bike and get to know it well before your century, than do that. Otherwise, reward yourself after the century with A NEW BIKE!
Congratulations!