Without knowing your actual fitness level, it is difficult to say when you might be ready to ride a century. There are a few rules of thumb about increasing your fitness level.
1. Do not increase your capacity more than 10% in any week. If you ride 50 miles (total) this week, next week you should ride 55 miles.
2. Add hills & intervals in your workout. "They" say that you should not do intervals more than once a week because it is taxing on your body & recuperative system.
3. Take time off so that your body has a chance to repair itself. One day a week is good. Once a month, you should devote one week to active rest, e.g., light cycling or another form of aerobic activity. That way, you don't overstress any part of your body.
4. Hit the weight room. For endurance, you want higher reps with less weight (reps should be 12-15 with the last few being difficult but not going to failure). Squats, leg extensions, lunges, hamstring curls, calf raises, etc., are good. Don't forget your upper body. A good website for beginners: http://www.billpearl.com/ Click on "programs" for some ideas.
This is a good time to be in the weight room. Taper your lifting as you begin riding more, but don't quit entirely.
Good luck!![]()