DeVanny and others have done the whole cancer to long term training link (aka marathon runners and distance bicyclists). Just looking at it from the health of the body point of view, long distance training certainly increases the level or cortisol, thereby decreasing glucagon (muscle saving), and also has high oxidative damage. Now all that stress to the body certainly can not be healthy long term. (I am referring to people who do LSD on a consistent basis and not someone who does it for fun on the weekend). If you look at most LSD trainees you will most likely find a higher % of BF, less lean muscle, increase in illnesses, cold, etc, depressed immune system, more joint damage, high levels on inflammation, increased risk of skin and other cancers, and also just a general aging to them (they look older). I think fitness should be concerned with overall health and balance too....otherwise why do we do it? I run/bike distances here and there...but it is certainly not a staple to my training program.
I usually say sprinters can have decent edurance over longer distances, but joggers usually are not good sprinters. Of course you train for your needs.