Sometimes the simplest thing is the best option and nothing is simpler than ruck marching. Taken from the military tradition of marching with heavy rucksacks, ruck marching is going for a fast walk with a load on your back.
It burns almost as many calories per hour as running with far less impact on the joints. Because it conditions you to walk with a load it is sport-specific training for backpacking and long approaches for rock climbing.
Start with 2 X 30-minute ruck marches per week with a load of 10-20 lbs. On alternate weeks either add 5 lbs of weight or 5 minutes of time to your workouts until you are at 60 minutes and 20-25% of your body weight on your back.