Extended cardio’s gotta go, people!
“Contrary to popular belief, extended extreme cardio, such as marathon running, actually sets in motion inflammatory mechanisms that damage your heart. So while your heart is indeed designed to work very hard, and will be strengthened from doing so, it’s only designed to do so intermittently, and for short periods—not for an hour or more at a time. This is the natural body mechanics you tap into when you perform HIIT.
Repeatedly and consistently overwhelming your heart by long distance marathon running, for example, can actually prematurely age your heart and make you more vulnerable to irregular heart rhythm. This is why you sometimes hear of seasoned endurance athletes dropping dead from cardiac arrest during a race. I ran long distance for over four decades. So please learn from my experience and don’t make the same mistake I did.
Compelling and ever-mounting research shows that the ideal form of exercise is short bursts of high intensity exercise. Not only does it beat conventional cardio as the most effective and efficient form of exercise, it also provides health benefits you simply cannot get from regular aerobics, such as a tremendous boost in human growth hormone (HGH), aka the ‘fitness hormone.'” Dr. Joseph Mercola
The Wellness Quest 