I've just passed the 20-year mark commuting to work year-'round in Chicago. My commute would be 50 miles each way if I rode all the way, and I've done this a couple times just to be able to say I did it. This is usually on Bike-to-Work Day, and I agree with Stuart that it's pretty much a failed experiment. Everyone talks about how many people ride in the city, but my observations tell me that it's only a small percentage of potential riders who actually use bikes. Perhaps I'm riding in the wrong areas; there certainly has not been much of an increase in "multi-modal" commuting.
Normally, I ride a folding bike to the local heavy rail commuter train line, then get off and ride the bike the last few miles. My total mileage is about 7-8 miles a day; not a lot, but it serves as my "base" miles for getting in shape for longer rides.
My workplace does not provide showers, so the short distance works well... I'm an endodontist and spend a fair amount of time in (very) close proximity to my victims patients, so I don't get too fragrant.
I figure I have about 25,000 miles of street riding experience after all these years, and this has helped me stay in reasonably good shape for my age (72).