Finally got a replacement strap for HRM so tried that out today. Still done have an in-shape cool-weather baseline but I'm in reasonable shape and it wasn't too hot out so not a bad place to start collecting info. Intentionally kept it very slow and only looked at HR the whole time. Was actually pleasantly surprised I was able to keep it reasonably low. I guess with cardiac drift you have to slow down to keep the same HR as a run progresses... at least, I don't think it was a coincidence how that happened. I'll try to use it consistently, we'll see what we get.
Anyway otherwise things are feeling good. Calves a little shaky from the sudden onset of heel drops but otherwise I can't complain. Granted I rarely have problems at this pace... but not never!