Drat. I woke feeling better after resting all night, but within about 5-10 min my leg was hurting quite a lot. It's very painful now (at lunch time) and I'm going to ice it. Coach's best guess is posterior tibialis muscle pain or tendonitis.
Today was a cross-training day, but that's not going to happen. If it's still this bad tomorrow I'll try the bike, or maybe swim. If it's a bit better tomorrow I will probably see if I can walk a little on the treadmill; if not, at least I can get right off and switch to elliptical or bike or whatever.
I'm still hoping this will only be a couple-3 day setback. If that's the case then all my hard training and the progress I've made will be preserved. Here's hoping!