I think the key here is defining the duration in question. Brief, high-intensity interval type training will produce adequate results to a point. But once you start getting into longer events, specific LSD training is a must. I don't think that extended training is necessary to actually develop endurance in the sense of cardiopulmonary capacity, but more to condition body structures to endure that kind of extended, repetitive abuse, develop movement technique and economy, and to prepare the athlete mentally.
Another component is simply body comp as well--to be a successful endurance athlete, you simply cannot be large and muscular. Long term endurance training will eventually cause atrophy of type II muscle and even type I eventually to create a body of minimal mass and great capillary density. If muscularity is maintained through high intensity training, this is obviously counterproductive to high-end endurance athletics.