It really is mind over matter. I’ve discovered in the past, when running outside especially, if I have a particularly good collection of music with a rhythmic beat or a song associated with action, running, etc… I just concentrate on the music and I don’t feel the tiredness that comes with a long run. Though to be truthful, it’s usually in first 10-15 minutes of a run that the thought to quit arises. Once I get to 30, 40 minutes my mind is on auto pilot. I won’t say that it’s the actual “runners high” but it’s pretty damn close.