I did a 3 week backpacking hunt last season. The one thing more than anything that I could feel the wear and tear on was legs. If youre carrying a backpack, or even a daypack, you will feel it in your hip flexors and legs. Squats, lunges, leg press, and lots of walking and some running will help a ton.
As far as diet, the more processed carbs, sugar and grains you can cut out of your diet, the more fat you will get rid of. Lots of meats, veggies and fruits. Dont be too afraid of fats, as they are essential for energy as well.
Another area to focus on is your back. Hiking and climbing can make your lower back sore. Strengthen your core. Abs,back, chest and legs.
you dont have to be a super athlete, but the better shape youre in, the more stamina you'll have. Keep hydrated, and while on the hunt make sure you fuel your body. I was at 178 when I got there, and 164 when I got home, and that was eating 4-5000 calories a day. Im 48 yrs old.
Dont over train and hurt yourself,even getting too sore will set you back. I walked a lot with my pack on, 40-55 lbs to train, did the treadmillat the gym at full incline and walked to train my legs, as well as the other exercises I mentioned.
P90X works for a lot of people, but its not for everyone.
Enjoy the journey. The anticipation and preparation as much part of th hunt experience as any of the rest of it.
One other note. While we all train our selves to be physically ready, there is one aspect to elk hunting that is hard to train yourself for, and that is mental toughness. Staying motivated when game is hard to find, when weather, and that mountain start to beat you down, when you have 3 more miles to go, and only enough in the tank to cover 2. You need to be mentally tough. ANd trust me, it sounds a lot easier than it is.
I dont know how long youre planning to hunt, but if its a week or more, dont be afraid to take a day off to rest your body.