As a keen hiker and backpacker, the best way to train to carry that kind of weight on your back is to .... carry that kind of weight.
Although you may want to first start out with something lighter - say around 10 lbs, and then build up from there. Moving at high speed with a heavy pack can be very hard on your shoulders as the straps jerk up and down on your shoulders - at least until you learn a smoother, more even stride.
The other thing is that a book bag may well be able to handle 25 lbs, but carrying 45 lbs is probably into the range where you need a framed backpack to help transfer the weight to your hips, rather than having it all hanging off your shoulders.
I'd also strongly recommend including planks in your strength training routine. This move is really good for your back strength, which is necessary to support a pack and keep yourself balanced and stabilized.
btw, those speeds seem quite fast for civilians carrying that kind of load. Are you sure you have the figures right (eg. were the distances in kilometers rather than miles?)
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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