In a world where danger lurked around every corner, your ability to run was a strong indicator of whether you would live long enough to pass your genes down to the next generation. (Note to Nietzsche: That which didn’t kill Grok made him stronger). Avoiding a charging beast to save your life, or surging forward to catch a different beast for dinner, the net effect was still survival. A combination of the hormonal events that occurred simultaneously and the resultant gene expression within fast twitch muscle made sure that the next time this happened Grok could sprint a little faster.
Do some form of intense anaerobic sprint bursts several times a week. This could be as simple as six or eight (or more) short sprints up a hill, on the grass, at the beach… or repeated intense sessions on a bicycle (stationary, road or mountain bike). These short bursts also increase human growth hormone release (HGH is actually released in proportion to the intensity (not the duration) of the exercise).