I completely agree about commuting. I started doing that this year and have really improved my speed. I'm in grad school and am working on a big research project, and also work part time to pay the bills. I'm very busy, and was finding in April that I wasn't really riding much. Grant it I live in Chicago and the weather was bad a lot, but I could have done a lot more. So I decided to start commuting in May. I'm totally hooked. There are threads on commuting tips so I won't get into too much detail here. But my average speed in April was probably around 15 mph. I was topping out at 18mph for any speed I could maintain for any distance. Now I cruise into work at 20+ mph. I was going 21.5 into a slight headwind yesterday. That's just with about a month of riding pretty much 5 days a week to school/work.
Good luck!
"Only the meek get pinched, the bold survive"