Thanks for the responses so far. I should have said it's a four mile commute by car - I think it's significantly longer by the route I chose, because I drive on 4 lane thoroughfares that are not safe or topographically logical for a bike.
The door-to-door time is a good suggestion. I'll check it out. I don't have to park far away - only about 150 yards, but the bike rack is 10 feet from the door. If I walk to my car really slowly, I bet I can make the bike commute more competitive.