Start rucking.
But 101stAirborneVet, I was never in the military! What is "rucking"?
Explained here, it is essentially walking quickly, over rough terrain, with a load on your back.
But 101stAirborneVet, why in the world would I want to do that? How is that helping me to "prepare"?
One of the basic tasks (if not the basic task) of a person who finds themselves in a combat situation is to move. That means getting from point A to point B, usually with supplies/ammo/weapons/correspondence/food/water, as quickly as possible.
But 101stAirborneVet, I'm out of shape! I might have a hard time carrying food, or water, or ammo a long distance - especially during stressful times when my body will already be weaker!
That's why you need to train up now. The First Implement is your body. If you're out of shape, start getting in shape now, and practicing realistic physical challenges like strapping a pack on your back and moving 12 miles as quickly as possible. (Work up to this, of course. The above linked web site has some tips on this.)
I'll see you out there.