Bad assumption. Horses only take you so far. You may very well find yourself having to make a quick 1/2 mile to a full mile jaunt to get into a shooting position. You won't be shooting from the back of a moving horse. Remember, this jaunt may very well be at 10,000+ feet while carrying at least a rifle and probably a daypack. Can you make that jaunt right now and still be able to shoot accurately the minute you get to where you are going? Be honest about this because elk can live in some really rugged country.

My suggestion is to go to something like a football stadium and jog up and down the steps with at least a 30 lb pack on your back and something close to the weight of your rifle in your hands. Don't forget to wear the same boots you intend on hunting in. Work on that until you can do it without becoming too winded. Then you can really start working out. Get to where you can do at least 30-40 sets of stadium steps (not single flights like in a regular building). You don't say where you are from but hunting the Rockies is going to be a lot different from what you are used to.