I'm going to recommend the fixed gear route too. It's a lot better on your knees. It's also a lot better workout, as you end up using your legs to slow down as well as speed up. My commute is only 3.75 miles each way, but I have found a couple of routes that give me between 10 and 20 for days that I want more miles. Simply exploring your city is a good option, too. Just make sure to have a timepiece of some sort so you're not late. I have a little travel alarm clock that I keep meaning to use for rides, set the alarm halfway between when I'm leaving and when I need to be at work. I have yet to use it though...