I am currently reading "Why Zebras Don't Get Ulcers" by the noted stress psychologist Dr. Robert Sapolsky, Ph. D. I came across this quote that seems to be just another good case for HIIT vs. long distance running...maybe I am taking it a bit out of context but I feel it still applies:
"This brings up a broader issue important to our era of lookin' good. Obviously, if you don't exercise at all, it is not good for you. Exercise improves your health. And a lot of exercise improves your health a lot. But that doesn't mean that insanely large amounts of exercise are insanely good for your body. At some point, too much begins to damage various physiological systems. Everything in physiology follows the rules that too much can be as bad as too little. There are optimal points of allostatic balance. For example, while a moderate amount of exercise generally increases bone mass, thirty-year-old athletes who run 40 to 50 miles per week can wind up with decalcified bones, decreased bone mass, increased risk of stress fractures and scoliosis (sideways curvature of the spine) -- their skeletons look like those of seventy-year-olds."
Its something I never really thought of before -- but the increased stress of 40-50 mile runs is downright degrading to the body. It does have proven effects especially in older populations.
If the goal is not to be elite, just to be fit, I think this is another good argument for HIIT vs. ET. HIIT can give you many of the benefits of long distance endurance training (plus some other good benefits) without the negative effects. This means one can run 12-20 MPW at most and not have to worry about the problems associated with severely long run times and thousands of impacts onto the joints from each stride.