One thing I found to be very effective : you know that miracle protein based paste for starving people in Africa and elsewhere (that I weirdly can't find here)?
Do some exercise and eat that stuff. It is designed to restore muscle tone and of course brings carbohydrates but while making exercise it is extremelly effective.
By exercise I mean you have too run and do something, not just staying there spraying. Even better if you have a "propper" load out. I used to have german and multicam version in France about 15kg (suck it Imperials :D), now with GBBR once my PC will be be propperly loaded it will easily be 20kg.

Record after a 24 hours milsim (non stop game) : about 1.2kg of extra body mass. And given how easy push ups and other exercise were afterwards it was not fat.

Plus you help support a useful product for once (says the game who makes video games). ;)

I don't about you guys but I wasn't even sprinting anywhere and I was soaked. A good airsoft match burns off as much as a basketball game for me.

Just my 2cents

Bumpy bump.
I just got a heart monitor (Suunto M2) I tried it under my body armor and it does not feel like it will be problematic.

Now I will wear it to the next milsim and post the results.

Long_Bong: Any results so far?

I don't think you will get in shape while playing airsoft, but it surely helps to achieve it. You need to play really often or do something else beside airsoft to stay in shape. It also depends a lot on the style of the player.

With full loadout body armor and medbag plus bullets etc. for a 4-6 hour patrol all my kit was weighing in at 95 pounds give or take 10 or so. That was A good workout not practical for airsoft but try it see how it goes just stay hydrated