You are spot on. I remember Mark Twight mentioned that shorter intense work provides adequate machinery for longer efforts but not the mental toughness. Only longer efforts can provide this. Some of the central governor theory (the piece Frank Forenich did for the PM for example).
It's interesting also...we have generally recommended CF type WOD's ~ 3-4 days per week, some sprint work and one long effort every 7-10 days for our endurance folks, particularly multisport. Some of these people are getting to a pretty high level of performance, placing well, improving race to race and avoiding most of the repetitive injury issues. Much more time efficient also.
Its interesting...after about 2 hrs of say a 70-75% VO2 effort the fueling shifts from intramuscular triglyceride (IMTAG) to blood borne free fatty acids. The body appears reticent to deplete the IMTAG too far and if FFA's are not readily available things will grind down. Increasing IMTAG's and ability to put FFA's into circulation is key for those longer efforts.
"Survival will be neither to the strongest of the species, nor to the most intelligent, but to those most adaptable to change."