Thanks all.

I do plenty of core work - including some yoga (not consistently, however) and circuit training. Perhaps I didn't warm up and stretch enough this morning. I usually do some light calisthenics before I run to get the blood flowing, but I overslept -- again...still figuring out a new alarm clock we got! -- and I rushed things to head out the door so I could get my run in before it got too late.

Back is mostly better now, for having spent a day moving around. Next time I do this - I'll be more diligent in my pre-run routine and see how that goes.