Re: Old dude heads west.
Skip all the fancy supplements. I live in Longmont, at about 5100 ft. I am also 48, or so, and when I moved here from CT, it was a little tough at first. I took a few days to get acclimated, despite the fact that I was routinely cycling close to 5000 miles/yr. The thing that kills you out here is your ability to recover and utilize oxygen at higher altitude (say 7-8000 ft or more).
If you start with some form of cardio training, you'll be ahead of the game. If you want to drop pounds, do some form of Atkins, where you limit the amount of carbohydrates in your diet. If you cut carbs, and exercise 3-4 times a week for an hour or so, you'll drop the pounds. This will serve you much better than weight training for hunting around here.
The only weight training I would do would focus on the lower body, and would be more circuit training. Lugging large heavy muscles around is hard work, and tone lean muscle works much better at higher elevations. Fast hiking is probably a good start. Also try some kind of "interval" training, where you hike fast for 3 minutes, at your limit, and then slow down and recover for 10 minutes. Do this a 4-5 times, later on in your training schedule (say april/may).
One more than one occasion, we have hunted uphill in the morning, trying to get ahead of the Elk that are heading back to bedding areas. We have found ourselves "hoofing it" up some pretty steep areas, and then suddenly, you spot the herd. Being able to recover from a harder effort and get a good shot off has been key.
When you do come out here, everyone will tell you to drink lots of fluids, but the key is to also consume some salty bars, or electrolytes. This will help you retain more water while hunting.
If you like your hunting rifle, you can keep your hunting rifle.