This one’s easy, and the answer may surprise you: CrossFit.
Because if all you’re doing is running, you’re not “spreading out” the exercise to the rest of your body, especially the core. Plus, working more muscles, as you do in CrossFit, also increases testosterone and growth hormone while decreasing bad stuff like cortisol and insulin.
Another great benefit is that CrossFit increases nitric oxide (NO) to a much greater extent than traditional cardio, and this results in bigger arteries, which is great for your heart, brain, and penis (yes, that means sexual performance). High-intensity power training (HIPT)—exercises like squats, dead-lifts, and overhead presses—boosts oxygen consumption, the most accurate measure of aerobic conditioning.
So, don’t hang up your running shoes, but mix it up to do double duty for your heart.