I suggest starting with the 40 mile route. When I started doing the ride for the roses three years ago I started with the 40 (then they called it 35). Last year I stepped up to the 70 and let me say it was a major step. This year I am riding 100. I suggest about 2 to 3 weeks before the ride trying to see if you can at least ride 30 miles. Keep in mind there are rest stops every 9 to 15 miles and since it is your first time make use of them. But ultimately enjoy yourself and take your time.