Excellent thread question!
I've only experienced a true runner's high twice, both on long runs of over 10 miles. It's the weirdest thing. Both times. It was hot, really hot, and I was exhausted and extremely out of breath, panting and heaving. (I probably should have hydrated better.) I kept thinking, "When will it end? Am I going to pass out? Should I stop and walk?" Then all of a sudden, within the next step or two, I felt like I was floating, like my foot falls were barely touching the ground. My lungs filled deeply and completely, and I had more energy, so much more energy, than when I began. I remember seeing the course I had run a bunch of times with new perspective. I'd always noticed its beauty, but somehow now it was more vibrant and clear, crystal clear, and my exhaustion couldn't cloud my view. Before the high hit, I felt like I was somehow punishing myself being that exhausted and out of breath, but then after the high hit, I felt that I wasn't running alone anymore, that God was running right there with me. I'll never forget it. It's a miraculous feeling, and I hope it finds you.