I live 3.5 miles from work, in a town that's one of the most bike-friendly in the nation, situated in a great climate, and I don't have any other time in my day for going to a gym. It really is a no-brainer. I used to live further (5.5 miles) and the difference in the ride time, tiredness and home time with the kids was noticeable. If I lived twelve miles from work I'd do something else. Some of the guys on this forum doing 40 miles RT, 200 days a year, I can't imagine.

FWIW, I live in the neighborhood nearest east of my job. There are closer apartments, but 3-3.5 miles is about the distance for houses. There other neighborhoods similar distance west and north. But that wasn't why we chose this house. We moved because we wanted to change school districts, and the former house was in a suburb that's going to the dogs. If I had wound up with a 12 mile haul, I'd have accepted it... and done something other than bike commuting.