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 on to the next generation. (Note to Nietzsche: What did not kill Grok made him stronger). Dodging a charging beast to save your life, or swooping in to catch another beast for dinner, the net effect was always survival. A combination of the hormonal events that happened simultaneously and the resulting gene expression in the fast-twitch muscle ensured that the next time it happened, Grok could sprint a bit faster.
Do intense anaerobic sprints several times a week. It 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 bike (stationary, road or mountain bike). These short bursts also increase the release of human growth hormone (HGH is actually released in proportion to the intensity (not duration) of exercise).