I think you would be best served to talk to the guide and even Len about expectations for the hunt. How much are you on horseback during the day, how far can you expect to hike, etc. The last thing you want to do is get there and not be physically prepared, especially after throwing that kind of money at a hunt. If they say be prepared to hike 6-8 miles a day, I would be sure you can do 12 at home. The weight of your gear, gun, pack, etc combined with the rugged terrain will make that walking much more difficult. The other thing to remember is the altitude. I live in the Denver area at approximately 6,000ft and when I get up to 9-10k, it makes breathing difficult. I can't imagine coming from sea level! All of this is why I would suggest being able to hike twice as far mileage-wise at home before the trip.
As far as training goes for the trip, stair steppers and running are good. Even jumping on a bike can help with getting your legs in good shape. If you can load up some weight in a pack and go on hikes or get on the stair stepper, all the better.
One other thing to mention as far as weight of your rifle goes: I would highly suggest not adding weight to it. An 8lb rifle can feel very heavy after trudging through the mountains all day, let alone a 12lb one.
Now everything I said goes out the window if you are on horseback all day and don't have to do much walking!
I just highly doubt that will be the case. Good luck!