Have you read the classic, "Cyclist's Training Bible" by Joe Friel? It's a GREAT place to start a training plan -- thousands of racers and riders have started there.

There's more to this than hours in the saddle. Duration, frequency and INTENSITY all play a part. Periodization is important. Your peak training hours should almost certainly be during your base phase training -- then fewer, but more intense hours during your build phase. How to start training? I would start with Cyclist's Training Bible. Follow that prescription. There are lots of books/authors out there who can help you modify and perfect your training plan, but CTB will get you off to a great start.