Fun & Fitness over 50

I've decided to ride a half-century on June 8th. It's the Tour de Cure in Stillwater, NY (about half way between Albany and Saratoga Springs), a benefit ride for diabetes research. There are 10, 30, 50, 62.5 (metric century) and 100 mile rides. I already do 25 mile rides every week so 30 would be no challenge and I should probably do the 50 before I do a century don't you think?
The sign up is on Feb. 27th. I almost backed out of it. I'm a little intimidated because I don't have a real road bike. It's a "mountain styled bike" as Guitar Ted would say. Not even a real mountain bike. It's a Ross Mt. St. Helens, rigid frame. My LBS guy, Matt, thinks it's not a bad bike and says I could put skinny tires on it. That would make it better but I want to keep the fat tires cause my wife only rides off-road with me and I can't afford a spare set of wheels although maybe I could get some used ones.
The more I think about it, what the hell do I care what people think as long as I can finish the ride.
Maybe I'll even wear a funny helmet so I stand out even more! ;-)
Yeah, that's it. I'll be a character!