U-shaped Curve to Exercise Benefits

Everyone agrees that exercise confers significant health benefits, leads to not only a longer life but a more enjoyable life.  Exercise has shown again and again to reduce one's chance of cancer, osteoporosis, heart disease, heart attack, hip fracture, and other such health perils.  For the longest time many doctors and professionals felt that more was always better.  For decades we have been told that marathon runners were the elite, the most fit among us, and thus the most healthy.
 
But the consensus against such claims has slowly been building from the minority for the past two decades or more.  Evidence that the dose response curve between exercise and health benefits is shaped like and upside down U is now mounting and creating legitimate controversy.  More doctors and sports professionals are now coming to favor shorter and higher intensity exercises.

Physiological studies are now showing that anaerobic exercise carries over to develop aerobic exercise but not the other way around.  From the perspective of the muscle cell and the metabolism of energy this makes perfect sense as cells produce ATP through multiple pathways but aerobic exercise rarely taps into the pathways required from high intensity anaerobic exercise.  Conversely anaerobic exercise taps out all of the pathways which metabolize energy and thus produce and more thorough "exercise."

Thus CrossFit and its now more than a dozen spin-offs (Seal-Fit, Military Athlete, OPT) both official and un-official are bringing the idea of high intensity short duration training to the main stream. 
The bottom line is exercise has a therapeutic range of values.  Below these values one should not expect results, while beyond these values more harm will occur than benefit.  This range differs between people and individual fitness levels, but in my opinion marathons fall outside this range for most people. 

If I spend more than 40 minutes conducting a work out and am not yet completely exhausted then the workout is to easy or I am moving to slow.  Generally I try to keep my workout challenging enough to wear me out in 20 minutes.