Get a weight vest, and do as much walking as you can, preferably on hills if you have them where you live. I've got a weight vest I can load with up to 60 pounds, it's a different kind of work than just running or lifting. Cardio without extra weight isn't going to help you much, the extra muscle might not either. Hitting the weights hard for 3 months also could end with you injured if you're not accustomed to it. If it was me in your shoes I'd just walk/march with as much weight as you think your pack is going to weight, possibly more considering the time of year you're going you might also be sludging through snow. I'd probably do that twice a day if possible. I'd also focus on core work to go with the marching, a strong core will really help you if you do a ton of walking on uneven terrain with a bunch of weight on your back. I'd work with kettlebells and sandbags vs doing squats and bench.