i am sorry mike but your WRONG, adding mass to a object/person DOES affect speed and endurance you can't get something for nothing, yes you might get used to the equipment, but its still there, adding extra weight, extra weight your muscles have compensate for, its the same as trying to run thu water, i don't care how long you been runing thu water, your not going to out run someone runing on land, its basic physics.
if you take 2 people of equal fitness, give one more weight to carry then the other, and he will be slower and have less endurance then the other, no matter how long hes trained with the extra weight.