I have only experienced it once in my life, so I consider people who feel it regularly to be pretty lucky. I would love to have more motivation and reward in exercise. As it stands exercise is a tedious chore for me.
Oh great, let's stare at the ground for the next hour. Let's trod the same ground, day after day. Yep, that pile of cigarette butts is still there. Oh look, the street cleaning crew missed that weird children's doll in the gutter again. This is so much fun. Let's do this for the rest of our lives.
I wasn't a heavy runner before, but I had trained for/completed an Olympic triathlon, and run several miles every once in a while like most formerly-athletic people.
But the first time I ran on a trail in the woods I was like "YEAH!" and I still get no similar rush from street running though I do that 10+ miles a week these days.
I usually try to run in parks, and usually I end up thinking about a bunch of stuff (work,daily life...), barely noticing the fact that I'm running, or where, or the music playing.
Also my thoughts during running tend to be much more positive and optimistic (to the point of daydreaming) than otherwise.