Well I wouldn't try and run fast when cold or tight, I would simply warm the body up very slowly and increase tempo. You will see lots of rugby teams warming up before games (especially 7's) with zero static stretching but rather jogging back and forth, then adding jumps, bends forward while jogging, etc. eventually increasing the pace to a sprint for short distances.
40 years old here and no real pain. Odd pain in areas where bones were broken and one of my knee caps at the top is starting to tingle a little which is concerning. Overall not real pain to complain about; however, I know if I sprinted at some point something would pull, could be within 2 minutes, could be 10 sessions later, but at some point more than likely a hamstring will go. Last time one went it was pretty bad with severe black and blue which eventually drained all the way down to my ankle as gravity took the blood a couple weeks into it healing.