Did my quick bike ride again yesterday. Took it a little faster and completed the route in about 17 min. My back feels OK today. I've been concentrating on relaxing my grip (to not strain my back) and just letting my legs do most of the work (novel idea, especially since I AM biking).
I think the extra exercise (Sunday morning volleyball) has really helped by letting other muscles get a workout. I have to believe that I did too much cycling a few months ago without doing much of anything else. That being said, I'm going to try to play tennis tomorrow.