My hamstring was tight

Even though I slept well last night, my wakeup HR was 48, indicating my body is still adjusting to normal sleep. After about a mile into my run, my left ham tightened up. I stopped and stretched it, but it was tight for another 8/10 mile. I stretched it again at my turn-around point, and it felt fine for the rest of the run. I slowed my pace a bit on the way back.