I am 51, live in New Orleans, and have elk hunted in Montana the last 5 years. Since I am at sea level and I think the highest hill in town is 24 ft, almost all of my cardio is done at an incline, usually the max setting. The best or closest to actually walking up a mountain is a step mill, it is much harder than a treadmill at 15 degrees. But if you're backpack hunting, I would strength train also. Getting an elk chopped up and packed out 5+ miles from some steep canyon is probably one of the harder things most people will ever do, especially if you add snow and alot of deadfall.
Like was mentioned in one of the earlier posts, almost all my strength training is compound movements, usually 3-4 sessions a week. I do cardio almost every morning for 40-60 minutes and in another month or will switch some of the sessions to rucking up and down the levees here with a 40 lb pack. I would not train any heavier than that with the pack, it is alot of stress on the body. If you can comfortably get up and down the mountains with that weight, you can pack the elk out. Figure with 2 people, 3 trips unless you really load heavy. I have done 4 now and really try to limit the load to around an elk quarter plus gear. Getting through the deadfall where I hunt is hard enough without an elk on your back.
The one thing I would stress is the better shape you are in, the more enjoyable it will be (and the better odds of success). We have had few people go that just underestimated (and did not listen) what physical demands are and they just ended up truck hunting after day 1. It is hard for many people in the east to grasp just how hard it is hike and gain elevation vs hiking on level land. Just add 1 or 2 degrees and you will feel the difference immediately. Add snow and many people are done after a mile or so.