The great thing about bike commuting is that because you do it everyday, or multiple times a week, you don't have to go hard each time, or the entire ride. I rarely do a hard ride commuting, but there are some days when during the latter part of the ride I'm feeling pretty good and I push it, and by the time I get home I am dripping in sweat, even when it's -10* and February. It's amazing what doing so little, but doing it everyday, can do for your fitness. Most days, though, I am happy puttering along at zone 1.