How did armies in the past survive ruck/force marches?

I know that during our revolution, boys to men about 70 survived 31 mile a day ruck marches, mostly with no shoes. I guess the trick is regulating body with lots of water and proper clothing. A huge chunk of it is pure will power. It should also be taken into account that a PT score of 280 doesnt even come close to the shape that men were in, in wars WW2 and before. Did you know that even the most in shape soldiers today barely ped the PT test from WW2? I doubt many of us are in good enough shape to storm Normany.