However, it's a little more complex than that. I did some reading about running and lower back pain. Possible causes are overpronation (rolling your feet in) and running up hills, among other things. It seemed to me that I might be overpronating a bit, so I grabbed a pair of mildly arch-supporting insoles from another set of shoes and stuck them in my running shoes. I also set the treadmill to an elevation of -.5 rather than 0, since I think 0 is actually a slight uphill tilt. Finally, I tried to pay attention to how my feet landed while I ran.
Verdict: I dunno. My back didn't feel sore when I got off the treadmill. It did a little while I stretched, and it felt pretty sore in the locker room. I drove home and then spent about ten minutes cleaning out the trunk of my car, which involved lots of bending over, and my back felt fine. I came upstairs and sat down at the computer and felt fine, but then I sneezed and that hurt like crazy. Now I feel not-too-bad again.
I should probably go get fitted for new running shoes. Or perhaps I should try running with lumbar support. Or... who knows? Maybe I should just revisit the punch line to, "Doctor, it hurts when I do this."
I'm taking my running stuff to Vegas but I may focus on core strengthening exercises while I'm there rather than running.