Find some local hiking trails, go to Home Depot and get a 40-50# bag of play sand, load her up and get moving up and down hills, no flat sidewalk. Then two bags for a while, when you get to three you are getting real. Get you a surplus duffel bag to load your weight in, it's easier to strap to your frame.
Most people think about climbing but navigating steep downhill on uneven terrain with a heavy load is no joke and a skill to be learned.
In the gym I use a Stairmaster Gauntlet, which will wear you out with nothing, but I wear a weight vest.
Lots of core exercise, planking sucks but it works. Legs may be moving you but the load hangs on your back.
There is a good series of videos on Youtube titled "Elk Shape".
Thinking about it won't work, gotta get moving.
Good Luck