I live in an extremely flat place with no substantial hills within an hour. I walk on the treadmill at about 2-2.5 mph doing intervals on the incline ie 5% for 5 mins, then 8% for 5 mins, then back to 5, then up to 10% etc. Usually for 45-60 mins. When starting I did this with 30lbs in my hunting pack but have worked my way up to 60 lbs. I don't plan to increase this weight for training as I will simply continue to increase the incline. I feel that limiting the weight will translate to less risk of injury with training.
I alternate the treadmill work with stair reps. What I do is do sets of 10 reps up and down the stairs. It is 16 stairs so each rep is about 10ft of elevation gain. I will typically alternate reps with single or double stairing (ie skipping one step). I make sure that for each stair, I raise my full weight on that leg up onto the stair before going to the next one, so it is more deliberate than just walking up stairs like normal. After 5-6 sets of 10 reps, its somewhere around 500-600 feet of elevation gain in about 40-45 mins with 2-3 min break between each set. With 60lbs in my pack I definitely feel the burn at the end.
Have also done "pack workouts" doing bench step ups, squats, split squats, reverse lungs with about 30lbs in my pack and typically holding 15-20lbs dumbells in each hand.
There's no excuse not to have some sort of routine. You can even just make it up as you go along, thats how i began doing the above. You don't need fancy gym equipment, expensive training programs, or steep hills. Just put some weight in your pack and move your body consistently for an ample amount of time.
