My first "backpacking" trip was a climb of Mt. Baker in Washington State - admittedly, not the smartest move in the history of the world. I had trained a total of zero hours for the trip and, as a result, I couldn't walk for two weeks after the climb. I decided that I needed to start "training" for my adventures - and thus, I embarked on several years of non-specific training, involving massive bouts of "chronic cardio" (i.e. endless running). Meanwhile, I wasn't getting any stronger. I had no lean muscle definition. I would always find the energy to actually climb the mountain, but the hike-out was agonizing.
It took time, but over the past 10+ years, I have figured out how to work smarter, not harder. I've morphed my training for hiking/climbing into a functional combination of HIIT (high intensity interval training), strength training and endurance. Over the past few years, I've finally been able to gain lean muscle and develop the speed and strength that I want/need to conquer bigger trips and taller mountains.
Disclaimer: I'm not a personal trainer (although I am currently working to get my certification). The workouts that I create are what I know works for me, after nearly 15 years of experimenting on myself. I share them here in the hopes that you can incorporate them into your own routine and find some value in them as well.
If you want more specific info, feel free to contact me, or join my private group on Facebook (click on the social media link below), which is designed to provide a more interactive forum for backpacking-specific skills, nutrition and workouts.