This is a real bugger of area as there are a few different tissues that need exactly the opposite treatment - one likes to be stretched, the other hates it.
It's either the insertion of your hammy or an irritated sciatic nerve. There's also a fatty pad called the ischial bursa that keeps them apart - the issue is if one of the tissues becomes inflamed, then so does the bursa - which in turn irritates the other tissue....and so on.
You need a great physio to tell you which is the driver of your symptoms, rather than the passenger. Tell the driver where to go and not the passenger...If you fail at GOOD conservative treatment or you want a quick way out (excluding the rehab) then get a US guided jab into the bursa.
Geri H sounds about right. Super common, but poorly diagnosed.
With your distance Woody is most likely that as you fatigue you're loosing control of the last bit of knee extension as your foot hits the ground. This forces your knee to hyperextend and stretch the tissues at the back of your knee - the capsule, hamstrings, popliteal bursa and popliteus.
Lots and lots and lots of inner range quad work (tiny knee bends, controlling the straightening bit) when standing. Lots and often! Ice and think about running with "soft" knees.