All my racer buddies who really do well swear by heart rate based training programs like those of Joe Friel's. He's got a couple of books that are specific to various sports. There are other people out there who coach in similar ways.

I've got his mountain biking book. I swear, some day, I'm going to get started on a real training program. I can never do it. I like to ride for the fun of it. I just can't seem to give up my "junk miles".

In the meantime, I've found that group road rides are a good way to push myself. Lots of fun, and learning how to ride in a group is proving to be almost as beneficial to my overall speed as outright improved fitness would be.