One things for sure - no matter how fit you are as a civvy, Military fitness will fck you till your body gets used to it!!
I know a guy who ran marathons and was a fell runner - fit as feck!! .........and while he managed a lot of phys with ease, put him in a pair of boots with a bergan on his back, or on his belt buckle leapord crawling in his fighting order and he was ball-bagged!! Its a completely different kind of fitness.