I've seen people complete a century who had never ridden more than 30 miles before. The ride is 80% mental! If you can, ride LONG on your weekends; maybe a 4 hr ride on Saturday (or whatever you can do) and a nice, easy (but fairly long) recovery ride on Sunday. Use those mid week lunch rides to remind the legs that you ride, and pound HARD if you can. Fit any other kind of fitness into your day that you can. Stretch. It all helps.
Make sure you have the nutrition/hydration thing figured out. That part is HUGE.
See how you feel when the day comes. Is this a supported ride? Will you be riding with other people? Amazing how much further you can go with friends (unless they push you too fast)!
You can only do what you can do. Good luck, but don't kill yourself!