My current commute to grad school is about 6 miles RT (or 12 RT depending on the location)...I've been doing that for 5 years. I usually don't bike in the winter, as it is cold, snows a lot, and they do a terrible job clearing snow here (especially on my side of town). Instead, I either walk, take the bus, or drive during the winter months. I've thought of getting a beater bike for winter, but really, I don't mind walking or taking the bus; even that small amount of exercise in the morning helps me get focused and makes me substantially more productive during my day.

In undergrad I biked around everywhere as well, which depending on where I was living or had to go to varied from 2 to 16 miles RT.

I've always found it was a nice excuse to get on a bike when I didn't necessarily have time to go out for a long road bike ride.