The better shape you are in, the easier you will find it to;
- attack positions
- performs maneuvers like pepper potting, advance to contact etc
- move tactically (leopard crawling etc)
Or you can just walk around and wait to get picked off...
Airsoft does not require a high level of physical fitness - if you can run 2.4km in less than 12min then i'd say you more than meet the general req's for cardio. anything faster is gravy.