Jilly - I'm not sure a runner's high is exactly about speed. I'm not a runner so I don't have first hand knowledge of it, but from the people I talk to it is more about time. Whatever speed they're going, whatever route they're doing, after about 20 minutes their mind just tunes into what they are doing. It takes them that long to get through the initial whirl of thoughts and all of that, and to just feel the breath going in and out.
