You know, I did a ride for the National MS Society two years ago, did a 20 mile ride across the Tappan Zee Bridge here in NYS, and I did that easily, probably one of the first 50 back out of I think some several hundred, I did that on a Trek 4300 mountain bike, and only worked out over the summer doing a 30 mile ride once a week. Of course, I played some football with friends as well, and skipped rainy weekends, but I did just fine on that. If you do a couple shorter rides during the weekdays when you get home or something, plus general workouts and a nice long ride or two on the weekends, you'll do fine. If it's an endurance ride, I'd suggest much more agressive workouts.