I would think a long ride up to 120 miles two weeks before, and maybe a century 2 weeks before that. So if you are able to do about 50 miles now, just ramp up the weekly mileage - 50, 60, 70, 80, 90, 100, (2 wks), 120, (2 wks), RAIN. Just one long ride a week, but also try ramp up your total weekly mileage in a similar manner. Be sure to recover after your long or hard rides.