I do a little of both. I don't run marathons anymore and I don't bench 300 lbs anymore....I am 46 years old, 5' 8" and 160lbs lbs. There aren't too many that can keep up with me in the field if I don't want them to. Key is consistency and making it a lifestyle. If it's important to you you'll find a way to make it work. No excuses, if you haven't already started, start today, not tomorrow. It's too easy keep saying "I'll start tomorrow."
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PS: If you plan humping a pack, humping a pack is how you train. There aren't any substitutes for it. You can do whatever you want to "get ready" but when you throw that pack on it's a whole different deal. I know guys who can run multiple 6 minute miles, however when they throw even a 50 lb pack on they start sucking. Granted any kind of cardio helps but put your hunting clothes on, including boots, throw that pack on and get out there. Even a treadmill works if you have to.