I don't remember who said it on this forum but it says it best : "Airsoft will not get you back into shape but you need to be in shape to play airsoft."
Any exercise that's not done regularly might be beneficial in the very short term but will not improve your health or shape much. You would need to have games (where you're really active) about 3 to 4 times a week to really have an effect and even Barbie doesn't game that much!
For my part I already train 3x a week and when I game I force myself to run everytime I move so as to maybe replace one training session (or add one).