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"Endurance training forces your heart and lungs to supply energy at 
continuous, low output, at relatively slow speed, for a long time. Long 
duration exercises tells your body to store more fat in order to meet the 
unnatural caloric demands you're placing on it by running all those miles. 
So when your body's at rest, it's busy getting fatter, not leaner.

"This type of exercise also puts your body in distress mode. It releases the 
stress hormone cortisol, which actually boosts triglyceride blood levels and 
cholesterol oxidation - both things that can clog your arteries and cause 
heart attacks.  Cortisol also lowers serum testosterone and growth hormone 
levels, which can lead to depression, decreased muscle mass, and clouded 
thinking.

A groundbreaking study of long-distance runners found that after a workout, 
"bad" cholesterol and triglyceride levels increased and threw blood-clotting 
factors off-balance, increasing inflammation and clotting incidence. These 
are both signs of heart distress and precursors to heart attack.

What you should focus on are high-intensity, short duration exercise 
routines. These mimic the natural physical demands our pre-historic 
ancestors confronted every day. Sudden, explosive bursts of speed - to 
escape danger or capture prey - gave them lean, fit bodies."