No amount of new gear will give you more success as having a solid Fitness Foundation. Not only you will have more success but also lots of more fun by not having to struggle with each step of the next hill.
Be CONSISTENT with your training; you can write down your training plan for the next six weeks and follow it. After your six weeks are up, evaluate and adjust your plan for the next six. Always challenge yourself a little bit more with each training cycle. There are many online programs that you can follow, and if you would like something customized with particular goals, Uphillathletes.com has coaching and customized programs design to the outdoor goals at needs.
TRACK YOUR RESULTS; the only way to know if you are improving is journaling what are you doing every day. This way, you can compare your results from week to week to make you are improving. In case you find yourself stuck in a plateau most probably it is time to change your training.
WORK ON YOUR AEROBIC FITNESS; it is an aspect that dramatically improves your performance when backpacking. Go for a one-hour run 4-5 times a week, low heart rate or zone 2. If you don´t have a heart monitor, you can use a nose breathing method to gauge your heart rate. It means running out of pace where you don´t need to open your mouth to breath. You want to do long and slow runs that will barely feel like you are breaking a sweat. If you have a knee injury that it does not allow you to run use cycling or swimming instead. Both are low impact activities and will improve your aerobic fitness.
RECOVERY; no matter how hard you work out, you have also to rest hard. Give your body time to recuperate what we build. You can even risk an injury or declining your performance if you push your body too hard without taking proper time off. Include Complete Rest or Active Recovery into your weekly routine. Sleeping is also a critical factor for recovery. If you are training hard, you need to get from 7 to 8hrs of sleep.
HIKE MORE OFTEN; it will be considered sport-specific training. If you want to get better going uphill with your heavy backpack, the best preparation is to go out there as often as possible and tackle hills. No training in the gym will replicate hiking in difficult terrain or stepping over the rock with a backpack on your back.
And if you want to check on your Fitness, where are you right now and what you need to improve, I prepared a Free Base Line Fitness Test Ebook, with the test and exercises.