Well, my 2 cents, I had been interested in airsoft for quite a while. Finally decided to get off my ass and do it. I am not tip top shape, big hills will kill me a bit, I recover fairly fast. That all being said, I got off my ass into airsoft to get out of the house, into the air to get some exercise.
Found out the difference between a good pair of "combat" boots and a huge pair of work boots to make a huge difference in the above mentioned hills, when you lose about eight pounds of weight per foot you are surprised how much easier moving is lol.
Not the best shape, but getting there because of airsoft. I know milsims require some ability to move fast, in extreme terrain and requires abilities such as hitting the dirt and the like. It's not for those who get winded after a flight of stairs, but for those who do, do not let it stop you, get out and do something start slow, let the enthusiasm of the game motivate you.
My first milsim I know I slowed my squad down, I told them to leave me behind and I would catch up. I knew my limitations. I did not expect them to hold up for my tired ass. I did catch up and did contribute. Great bunch of guys.
Again, I ramble. Yes, you need to be in some shape, but on the other side, playing milsims will help you get in that shape.