I just did a backpack caribou hunt in the Brooks Range. I started training 18 months prior to leaving. Goal was to put on five pounds of muscle, but ended up losing ten pounds during training. I am over 50 and I mixed in cardio, weights and 2X per week I would walk hills or an inclined treadmill with a 40 pound weight vest from Run Max. Did all this 6X per week with one day off each week for recovery. I started with 30 pounds in the vest and worked my way up. By the time I left for Alaska I was in the best shape I have been in since high school. The tundra was hard but the combo of the training types paid off for me. I wasn't about to carry 90 pounds uphill over tundra for two miles, but I was able to hunt hard and pack out my caribou without feeling like I was going to die.