Extreme endurance cardio, such as marathon running can actually damage your heart. The result can be far from what you would expect from an exercise program.  Research indicates that after 40-50 minutes of vigorous exercise per day, the benefits from your workout plateaus and additional vigorous workout time can even diminish the benefits. Additional vigorous workout time on top of that, can actually become detrimental.

Extended extreme cardio increases inflammation that can damage your heart. Your heart is designed to work hard and will gain strength by doing so, but everything has its limits.  That limit is less than an hour of extreme exercise at a time for most individuals who do exercise on a regular basis.  Even drinking an extreme amount of water can cause a serious electrolyte imbalance.

According to Dr. Joseph Mercola, the ideal amount of running for maximum health seems to be in the range of 10-15 miles per week.  Once you exceed 25 miles or more per week, the benefits actually disappear.  Running faster than 8 miles per hour or more than five times per week also can negate the health benefits of running  The best survival rates among runners are in those who run at a slow to average pace for a total of 1 to 2.5 hours per week, divided between two to three runs per week.

Veteran endurance athletes have a 500% increased risk of atrial fibrillation, a dangerous irregular heart rhythm.  Worse yet, some endurance athletes also present ventricular tachycardia, which can lead to ventricular fibrillation, which is a leading cause of sudden cardiac death.

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