Yep only experience breeds knowledge, im sure he will come back more educated. 25 miles on 70lbs is good going. What you say about mountaineering reminded me of the people who do high summits, usually they only hock the stuff into base camp. People go on extended trips into the woods carrying more, they stick to the level and i wouldnt have thought they do much more than hike into a camp or cabin, to carry it and them move on the next day is exausting.
I just did 3 days with 45 pounds of kit (45 to start, kit got wet 45 to finish) with the idea of educating myself, on the the level its ok but uphill weight is weight, and suprisingly downhill is almost as bad. This is from an unfit start, but its not something i would wish to train myself to do. After 2 days the effort left me tired.
As for the injuries, many many people habe told of the bad knees, bad ankles that they developed in there youth and are still in evidence today 40 plus years later. My advice is to wuss out at the start of trouble, as these stresses catch up with you. I do not know how people used to do it in the days of canvass tents and rucksacks without hipbelts, even the feather bags where heavy, blankets to lie on. I wonder how many miles they made a day?