Firecracker 5K

I am registered to run in a 5K race in downtown Ann Arbor Sunday morning (July 4), called the Firecracker 5K. (It was formerly known as the Tortoise and Hare 5K, after the store that sponsored it. Its new name reminds me of the Firekracker 5K I ran in Ft. Collins, Colorado, on July 4, 2008 - the day before Christy's wedding.) But this week I've developed lower back trouble that has kept me from running for a couple of days - as I write, on the evening of July 1, I am sadly dubious about the prospect of being able to do the race.