For me, commuting by bike is just another way of getting to work. I know I won't go to the gym after work (and especially not before work), so doing a 6 mile ride after a cup of coffee is a good way to wake up and enjoy the roads before too much traffic shows up. I live in a hilly area, so I'm going to have to push up some hills one way or another, but I don't try to set any records.

If I go a little faster, I either had a tailwind or I'm actually getting fitter. Usually by the end of the day I don't want to do anything other than go home, so any errands I run have to be on the way.

Commuting in the morning is head-clearing, blood-pumping priming for the day. Commuting in the evening is a cross between head-clearing and death-avoiding by the increased traffic.

I know guys that will go 25 miles after work. I'm not one of them, but more power to them.