I haven't been a runner in over ten years, but I ran a lot of 5k, 10k races and a marathon. Training to reach the goal of running the marathon was the prime motivator for me as was running longer and longer distances in my runs until a ten mile run as a routine was nothing. I got to the point where I could run that kind of distance at a good pace breathing through my nostrils the entire time if I was running on a flat grade.
The "runner's high" is definitely real. From what I recall, after 45 minutes of continuous vigorous exercise, the brain releases endorphins. I became accustomed to the runner's high to the point that when I was injured (ligament) and had to immediately stop running, I went through a withdrawal of sorts.