I've always worked out since i started in high school just to keep in shape and so I'd have an advantage playing sports also for my firefighting program. In airsoft it's important to have cardiovascular endurance and if your carrying heavy gear to have the muscle stamina as well. Just stay consistent and keep progressing to see the best results.
If you want to get in shape you can check out these sites for good resources
To get the most out of your training time stick to a nutrition and supplement plan(stick with lean proteins, healthy fats and complex low G.I. complex carbs and as far as supplements go it'd recommend at a minimum a multivitamin and protein shakes)
If your new to exercising/lifting weights I would start out with a moderate intensity cardio program and stick to compound movements :squats, dead lifts, bench press, bent rows, military press etc..if you combine that with sports training, full body stretching, sports specific training and plyometrics it will compound your benefits.