Some of the information that comes from some "Army" sources is a little suspect, especially with pack weights. One "ex SAAS" salesperson that worked for a company I worked for quoted ridiculous weights and time for the 10 kilometre run. Basically a SAAS platoon with army boots and 50 kilograms packs would have taken out the top ten placings had they entered a team in the Burnie Ten so me thinks more than a grain of salt is needed with certain claims. Also some of the other information was more in the line of Bollywood fiction, by that, even more unbelievable than Hollywood, regarding accurate weapon range. Military specifications tend to be rather overstated by some with 50 knot super aircraft carriers, etc.
I have no doubt that at times Army units in emergencies have carried extraordinary weights but find it a but dubious that that is standard practice.
That aside, if you train to carry certain weights then the body is able to adjust, just helps being young not middle aged or older

Cheers Brett