3-4 days a week go on a jog. Every time increase your distance/speed. Drink plenty of fluids and make an effort to eat better. Mix in pushups and situps/leg raises. You can also weight train, but you don't need weights. Weights are for strenght not endurance. Also stretch your muscles.
I do this pretty much year round. Even have my lazy weeks here and there though. I change over 9000ft elevation from where I live/train to where I hunt big game. It's always more challenging in the mountains. You can train year round, but nothing can train you for stepping on rocks, holes, walking through unforgiving canyons, and breathing at that height. Although I imagine it would be even more challenging if I didn't keep up with my fitness.