I think Melody is right that you'll feel REALLY different on the road bike vs. a 1 speed. If you can do hills on a 1 speed, I bet you're in better shape than you think.

In addition to proper training and adequate food and water during the ride, I'd highly recommend having bike shorts for the 35 miler if you don't have them already. That's a long time in the saddle!

Go for it!