I get runner's high occasionally.
I still remember the first time I got it: I was near the end of a long run in the hills and everything suddenly seemed to be so easy
: My stride lengthened out so that I felt like a gazelle loping across a field, my breathing became effortless, and I just felt really good. Mostly though, I remember thinking how easy it all was--effortless. Like I was floating along in a dream.
This went on for a minute or so until I realized I was experiencing runner's high....then it faded.
Since then I've had it a few times. It comes on unexpectedly, lasts for a few minutes, then fades. When I realize it's happening I just try to bask in it while it lasts.
I don't do anything to try to get into that state: it just happens.
When I do get runner's high I'm not doing anything unusually difficult or pushing myself any harder than normal. It just seems to come on at random times, with no explanation. It's sort of like my body is telling me: "Thanks for all the hard work making me healthier! Here's a little surprise gift for you in return!"
I don't try to force it or make it happen, though.