I'm lucky to have been able to hunt elk 15 of the last 16 years(two years I just called for friends). We usually camp at 10,000' and hike as high as 12,000 and as low as 8,500'(I live at 1,000') with long days hiking up and down and some 15-20 mile days (what's a horse?). I started the first year by running 10 to 15 mile weeks and got my hat and some body parts handed to me. I did kill an elk after 3 weeks of pain. Later I tried more running, up to 50 mile weeks and getting to camp early to aclimate. Better but, I have learned that it takes more than just cardio. Your legs have to be strong and conditioned. I got a hip sled a year ago and worked up to 600lb squats along with all the cardio I could stand and had the best season ever last fall. Packed out two elk of my own and helped with three others and my legs never did get sore (not, "where is the ibuprofen and somebody help me put on my pants!" kinda sore).