Being ready "just in case" requires the whole system to operate. If there's anything gunfighting with paintballs has taught me, it's that you will become intimately aware of every way your body isn't up to the rigors of battle very, very quickly. However far you can (or can't) run, how long you can hold yourself in the weird positions needed to take cover behind obstacles you'll actually find in the real world, instead of nice oddly shaped plywood cover analogues, how long you can carry all of the equipment you need for it, even little things like where your shoes rub that you don't notice during day to day wear.
When you make sure a car is ready for a big trip, you don't just check the engine. You check the tires, the brakes, the fluids. Any failures might not be anywhere near as major as the engine going, but will make the trip far less pleasant. Same for things really going wrong. The gun is the engine, you absolutely need it for a gunfight, but you're going to need the muscle strength and stamina and some training if you want to get through the whole event as well as can be managed.
Soon as I finish it up, I'll toss up a link to my Tactical Workout, developed based on what I've learned from major scenario games (a giant paintball war, waged over hundreds of acres. At Legends, which I still need to post up more about, I probably hupped 30-40 lbs of gear over ~10 miles, a lot of that at a full sprint)
No good to tell you to train your body and not do anything to help with it, right?