The problem is a bit definitional. What is endurance exercise? It seems that in almost all discussions, it is either running or biking for a sufficiently long period of time. How long? It depends, but usually the minimum is about 30 minutes at least 3 times per week. Less than that amount is seldom considered to be endurance exercise. And less than 30 minutes is usually not thought to bring adaptations that would be sufficient to place one in the endurance exercise category. You would not ordinarily become proficient enough at it to be considered an endurance exerciser.
Does endurance exercise give one endurance? Only if you do enough of it. Is it the only way to build endurance? No, one can achieve high levels of endurance by sprinting in brief intervals. Endurance exercise is not necessary for building endurance and it may not be sufficient for many reasons but primarily if the volume is not sufficient.
Then again, what sort of endurance do you want? Is endurance the ability to "get through the challenges of life"? An endurance exerciser may not have any more of that ability than someone who does a different form of exercise. For example, endurance for an NBA basketball player is a matter of being able to recover quickly from power moves of high intensity. Endurance in baseball may be the ability of an MLB baseball player to retain power and last through a long season. On a more mundane but more important level, endurance is the ability to get up every morning and do a hard day of work. An endurance exerciser may so exhaust his or her adaptive capacity as to reduce this more important measure of endurance. An MLB player who ran 30 miles a week through a season of baseball would likely end up spent by August and sitting on the bench.
A softball player I played with was an endurance exerciser, a good runner at that, who could not sprint to first base or hit a ball out of the infield. His endurance exercise depleted his power and speed, but he could run around the bases all day long. Not so useful in that aspect of his life and likely not so useful in his daily life. Will he have the capacity to recover from the hip or knee replacements he almost surely has ahead of him?
All this is just to add some complexity to the issue that I think is treated too simply by many. I am not questioning the value of exercise, which is considerable, but I am questioning the form and the amount. Not so new perhaps, but here is the different part of the picture I want to bring out.
Running or biking long distances has a very short and shallow curve of benefits before it turns and becomes damaging. Beyond 30 miles of running per week, the running damages health. Running more than 30 miles a week will increase running endurance, but it will increase the risk of organ and cardiovascular damage. See the link on my Facebook page or in the WSJ here. Now consider the shape of the benefits curve. No one knows where it peaks and no one knows the benefits curves of either running (or biking or a pure endurance exercise of some other form) or short intervals of more intense running, lifting weights, or of some form of anaerobic exercise, which is not considered to be endurance exercise.
Anaerobic exercise confers strength, lean muscle mass, and endurance. Endurance exercise confers only the latter and usually alters to body composition so as to shed muscle and cause the loss of strength.
But there is more to consider as my friend Nassim Taleb likes to recall. Of the hours he used to pedal to work he concluded that the hardest climb during the trip gave him most or all of the benefits of the considerable time he spent cycling. He and I prefer convex exercise where the upward curvature of the benefits curve created a situation in which the average benefit lies above the benefits of the average effort. Non-endurance exercise is naturally limited by the capacity of the system to supply energy and oxygen. Up to the point of exhaustion, the curve remains convex.
Endurance exercise does not have that sort of convex benefits curve. The benefits are concave and have a shallow peak.
Mathematically, you are better off going intensely and letting the body's protective mechanisms keep you from overdoing it. Not that I ever exhaust myself, which is mostly a matter of supplying enough oxygen to maintain high energy output and preventing the accumulation of lactate to the point of limiting power. This keeps you in the convex part of the curve and, provided you do not overdo it, has a better chance of creating the highest benefits of exercise. I never overdo it because I not seeking endurance or anything else. I have no goals other than to realize the benefits of exercise --- more endurance, strength, adequate muscle and high capacity.
So far, not so surprising and not speculative at all, I think.
Now for a more controversial and speculative conjecture. I think we now know that anaerobic exercise is not inferior to aerobic exercise and may be superior. I think it is. But, I do not promote a particular point of view on that as I think exercise is important and fun. But, endurance exercise is more likely to be harmful than helpful relative to the anaerobic alternatives. Again, I am not comparing NO exercise to either form. NOT exercising is harmful. But, so is endurance exercise.
I am putting forth the proposition that endurance exercise is inherently harmful. For beginners, it is inherently harmful because it is easy to overcome the oxygen delivery to tissues and ischemia will occur in some tissues. There are many warnings on running sites advising beginners to "go slow or easy" pointing out the dangers of inadequate blood and oxygen supplies to some tissues. There are also warnings against progressing too rapidly. But, how is one to know? You do not have ischemia warning signals that I know of that will protect you. And, how is one to know that you are not overdoing the volume? This is easier since you will show elevated cortisol and an elevated heart rate if you are overdoing it. It is the volume that kills because it leads to lessened immune function and accumulation of cortisol (a contributor to damage in many tissues and a factor in aortic function and damage that can progress into aneurysm and aortic dissection, extreme events that it may take years to progress into). Body builders and older persons training on weight doing 3 sets of 10 reps at the 80% of their one repetition maximum may also show elevated cortisol (some cross fitters also show elevated cortisol as I learned from Robb Wolf).
I think it is the volume that kills. This can hold for any type of exercise. So, aside from a few overzealous body builders and over-trained seniors following a similar level of volume, it is the endurance exercisers who are more likely to see damage from their exercise. And, the least benefits but for stylized measures of endurance that do not relate so well to the natural world.
I will have to think more about that to fashion an evolutionary explanation and even that may to too stylized. But, I think it is safe to point out that, without sophisticated testing, it is far too easy to enter into the region of harm because endurance itself becomes a goal and there are few, if any, warning signals to all but the very experienced and informed endurance exerciser. Evolution would not program such a warning system because there was no need for it. Natural exhaustion would occur first and it would call a cease to the danger in all but life-threatening circumstances where the warning could not be heeded any way.
Anyway, don't hate me for this. I am just posing questions. And, the evidence does hint at this conclusion.