Depends on what you want to do. I found this web site last week that has a chart to help you prepare for your first century ride, this is the same type of program that has been around for at least 30 years when I first used it. I hope it as some value to you. You could also shorten the miles listed by half or more depending on your fitness level, so you can alter it anyway you feel like. It is just suppose to give you a goal and a very real way of reaching it.

http://www.bikerideforthefamily.org/.../a0000038.html