Time-Crunched training plan by Carmichael.

Pretty good results with no more than 6 hours per week of riding, even for competitive riders.

It's hard though. Prepare for the pain after the 2nd week. I'm pretty serious about training, but after I caught a cold which derailed me 5 weeks into the program, I just couldn't get back on the bandwagon - it was too hard unless I started over.