This is a great thread! I was inspired to start biking to work with MMM's "How to bike all winter long" gauntlet throwdown, and the unusually warm winter we've had in Boston. We are a no car household, using strictly public transportation. This is pretty cost-effective, but we pay a premium to live in Boston and close to the T. So, I would argue being carless here is as Mustachian as someone who lives in a place with low cost of living and has one car.
I actually love the convenience of public transportation, even though it's a popular thing to complain about in the city. But due to transfers and indirect routes, my commute to work was about 1 hour long via public trans (for only 5 miles as the bird/bike flies), and for years I would wake up and go to the gym beforehand. So, 5:00 AM wakeup, breakfast, gym at 6:00 - 7:00, on the train at 7:30, at work at 8:30-ish. Then at night, I would crash way early to repeat the whole crazy morning again.
Then a eureka moment: though my morning commute & routine was healthy and relatively low cost, it could be a lot more efficient. I found a great bike route to work that takes me on bike paths past museums, through parks and a university. I also invested in some good lights, reflective vest, etc., and have started biking to work every day! It takes me 30 minutes instead of an hour, and I save an hour every day by not going to the gym. So, that's two hours a day, ten hours a week, 490 hours a year saved!!! Add to that the money saved on gym membership and train pass ($130/month).
So far no gym has been fine, 50 miles biking/week + weekend activities seems to more than do the trick. I was always worried about weather and being sweaty at work, but my new job there is actually a shower, and just as MMM promised, 30-ish degrees is no problem with a few layers (underarmor, sweatshirt and windbreaker).
Anyways as a new-to-bike-commuting guy, one observation I have is you VERY quickly learn, through necessity, how to keep your bike in good shape. Bikes were always a big mystery but now I am getting all the necessary gadgets to do regular maintenance and tuning on my own. Jumping from 100 miles/year to 50 miles/week, everything basically fell apart (riding an SE Draft, so really an entry level single speed, cheaper parts). I have the tools coming in the mail to replace my chain and freewheel, and can replace a tube or tire and tighten my brakes. Is there any other regular maintenance I should be doing do keep things from falling apart?
Also, at some point I'll need a better bike. Any recommendations for good single speed commuter bikes in the $200 - $600 range, let me know!