Good call .. don't mess with hamstring
normally, I'm good if i do full warmup w drills the day before, which i did, including yoga stretches. as it was, I walked about 15 minutes, then ran at the track for about 15, did minimal stretch routine, felt good. but I should have at least taken a hot shower before leaving in the morning. having run quite a bit in the preceding days, i was beginning to get creaky .
but actually i had a warning i didn't heed. just before the intervals, i felt something bad in my knee, several times. even asked someone if they thought it could be a knee ligament. shook it off and forgot about it. but it was actually the hamstring attachment to the back of the knee, ready to pop. Medical mystery solved!