Those times when I've achieved runners high it was when I pushed myself to carry a heavier load, and walk faster. Never during endurance training.
I would feel my legs burn and there's a temptation to slow down, but that was the signal to forge on commit to walking faster.
At the time I had a very specific motivation for walking, and I wish I hadn't stopped walking when that motivation was gone. It's hard to start up again.
Thank you for sharing.