The Problem With High Intensity Interval Training

It's going to lead to some physical burnout, mental burnout. It increases the injury risk, especially if you're doing them for a long period of time. The ultimate litmus test is are the best runners in the world doing CrossFit endurance? I don't know about from Tim Ferriss's four hour body. And it's developing a different type of fitness that won't carry over very well to a solid marathon performance.