It's pretty much basic muscular physiology. Same as lifting weights, most people don't go into the gym and just gut out a 2x bodyweight bench. They build up to it, preferably as gradually as you can, so that not only the muscles build up, but the tendons become stronger and the bone structure becomes more dense to support the target weight. Doing this take a good amount of time for the body to adapt.>>1544701
Are you training to be a grunt? If so, they rarely use the ALICE nowadays, you'd be better off with the MOLLE system. Just remember that the standard weight it the pack is going to be between 45-65 lbs for a dry weight, a lot of the additional weight will be from the rifle, vest, and rations so just try to train up to 50 or so. Keep good posture, it should feel as good as walking down the street.