Runner's High is also called The Zone, and it's very similar to many other things that are called The Zone, including that hyper-focused state of consciousness that programmers can achieve, and that semi-hypnotic trance that lets you drive for 6 hours on I-5 from SF to LA without going mad. It doesn't happen every time, depending on mood, stress, environment, sunspots, etc.

For me, it has a lot to do with rhythm. I find a good pace that lets my breathing and my steps achieve some sort of parity. Then I let my actions become automatic. It takes a little while, your muscles have to reach some sort of equilibrium, you have to get a feel for the surface you're on, and my mind usually needs to dump some useless junk first. Eventually the buzz dies down to a drone and you don't think about anything except the motion of your limbs and the air in your lungs.

Find a back road with no distractions and very few other people. Some people like music, some don't (Grateful Dead is popular). I find roads absolutely impossible, they're a erratic pile of breaking concentrations held together with joint-destroying concrete. Find a pace where you don't have to stop. Find a trail long enough where you can forget in the middle how long you've been running and how much longer you have to go. It may take a while. Exhaustion helps.