The real runners high
Runners high is caused by the release of a naturally occurring painkillers called endorphins during exercise. These are released at different levels in different people. They probably date back to the early development of human beings and the fight or flight response to danger. They are responsible for a feeling of euphoria and well being after intense exercise.
While it is different for all people, for me it starts as a feeling of "pins and needles" in the top of my head and radiates to rest of my body. Kind of like a pan of ice cubes running down over your head on a hot day. It may occur several times on long or hard runs.
Nothing better describes this feeling of euphoria than when you are finishing a long hard race, running down a long straight street with an excited crowd on both sides, children putting out their hands for you to high 5 them as you run by, and you put on a nice sprint and cross the finish line.