From what I've read, the 'runner's high' is quite uncommon and most likely experienced during, or after, an endurance event, but can also occur when pushing yourself almost to the point of collapse in a short, intense effort, such as running a 5km run.
That's what I did today. I did a quick tempo run and by pushing really hard I managed to get an under-30min-5K for the first time since returning to running following injury (and got a PB to boot!).
On the one km walk home I really felt euphoric. I'm sure it wasn't just the psychological boost of managing to run the distance in the time (and I didn't even know it was a PB until I got home and checked my stats), but was the endorphins. It was sleeting and the wind got up as I walked home, but I felt numerous positive feelings: I was conscious of just how warm and dry I felt inside my running gear (almost cosy); I felt happy I'd be returning to a warm house; I felt joyful that I could run outside no matter the weather; and I felt glad to be alive.
A great feeling.