I don’t know of any female backpackers who carry 40kg on one of their treks in the U.K. Even 25kg is unlikely.
Include consumables of food and water for a few days and packing for bad weather and off the beaten track and it's a very different story.
I'm not suggesting it's the norm (rather than "hundreds and thousands ... in the UK and worldwide" that would be "hundreds of thousands", at least) nor am I suggesting that any but a very few of them would have any interest in joining the Army, but it's a far from unreasonable weight for fit women to carry at those speeds over that sort of terrain over those sort of distances. It really isn't 'gladiatorial' or super-fit.