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.