I did a fixed gear century in Door County 2 weeks ago. Put in lots of miles before -- 30 to 40 mile rides several times a week, preferably in terrain similar to that in which your century will be done. A couple longer rides, 60 to 80 miles, in the month before the century will give you a good idea of your preparation. Don't overdo it in the week before the century, but don't slack off, either. If it's an unsupported ride, make sure there are places where you can refill your water bottles and get a snack. Take a brief rest every 30 miles or so. Not too long or your legs will get stiff, at least my 57 year old ones do. Don't try to set a speed record; the goal is to finish. Enjoy.