I always cringe when I see folks post up their 6, 8 or 10 week "hunting program". To stay in mountain shape a person needs work out year round imho. Building an aerobic base is measured in years, not weeks- so a person needs time on his feet, lots of time.
I'm blessed with no time constraints (the benefit of being retired!)- I'm hiking daily and strength training twice a week. I try to get in several backpacking trips outside of hunting season. The long miles and elevation gain/loss on these trips very closely mimic backpack hunting (sans the 80+ lb pack outs

). They also give you a great opportunity to get your gear dialed in.
If you don't already own a pair, get some snowshoes. Great workout- much more labor involved with snowshoeing and it hits a ton of little accessory muscles, ligament and tendons as well.
Hit the mountains, as much as possible