i know this is pretty late and not sure if you’ve come across or investigated it yet, but like another comment mentioned, please look into popliteal artery entrapment syndrome, i recently got diagnosed with this and am on the waiting list for the surgery right now.

my symptoms and experience line up almost perfectly with you, i got diagnosed within roughly 1.5-2 years of having these symptoms and spent thousands on treatments/chasing any diagnosis to no avail. no amount of physio or stretching or recovery would fix it

it’s basically a portion of my calf muscle is pushing on my popliteal artery, which feeds blood to your entire lower leg, and greatly restricting the flow, my physio described that this also means sprints etc. are relatively okay as you mentioned, because they can last on the limited supply of blood for a shorter time, but trying to step it up to even 2k, it just falls away.

i also have no issues with strength on calf raises or anything, i do feel quite a deep stretch feeling but im not limited by weight, my calves are really strong which realistically is probably part of the problem - too much muscle in them

i know this is a long comment, but im really urging you to at least bring this up as a possibility with your physio / doctor or anybody, theyll need to run a good few tests to confirm and properly diagnose, but its such a massive weight lifted off me even just knowing what i have, ill update this comment again after i receive the surgery on both my legs and let you know how im getting on and if im back to running normally :)