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.
2007 Seven ID8 - Bontrager InForm
2003 Klein Palomino - Terry Firefly (?)
2010 Seven Cafe Racer - Bontrager InForm
2008 Cervelo P2C - Adamo Prologue Saddle