"we were, on average, not designed for long distance running."
Tell that to the Kenyans and Ethiopians. Or even a lot of Westerners. Whether or not people are predisposed to sprinting or long distance running is, to a considerable degree, a function of where their ancestors evolved.
" We were made for sprinting. We hunted in groups that would sprint after food sources and sprint away from prey. Sprinting is distinctly aneorobic. Sprinting is, in essence, an interval."
We also hunted in groups, or as individuals, that would patiently run slowly after their prey until the prey dropped from exhaustion. No human ever out sprinted a deer or antelope. Quite the opposite, we ran after them slowly and forced them to start running again before they had recovered from their last sprint. Over a day, or more, the recovery deficit accumulated until they were unable to continue.
"Long distance, hard running has shown to drastically reduce the production of testosterone in men, while at the same time increasing cortisol levels."
Run that one by my wife. ;o)
"Interval training, which is proving to provide similar if not better cardiovascular results (and endurance) than constant running does the opposite."
Actually, a well designed program of longer distance training, hill work, intervals, and REST is the best way to train, if you are a serious runner. If you are just looking for decent cardio vascular fitness with a minimum of time and effort, interval training may suffice, but if you want to get really fit cardio vascularly, and build endurance for extended efforts, you will have to make the investment in time and energy that distance training requires. There are no shortcuts, even here in America. I have yet to hear of a marathoner or ultra marathoner running a competitive time on intervals alone.