I used to cycle 15 miles or so each way to work a few years ago. That place did have a fitness center w/ showers on campus, so it worked out ok (except having to get up an extra couple hours early). My current job just moved into a new building with a small fitness room and showers, so I may try it again next summer (about the same distance).
The best part was actually looking forward to my commute -- even though I only cycled about once or twice a week. Now I'm so far out of shape, it will take a few months of working out in the gym to get back to what I was doing (and hopefully my knees won't go out again).