Do you 40 something pluses have issues with fitness when playing. Doubt I'll be doing much running, the odd short burst of jogging might happen from time to time
Depends on the game scenario, the theme of which to some degree depends on the site (more casual vs more tactical sites). Some sites do games where both teams are supposed to run like madmen to the centre of the AO to grab a "bomb", press the buzzer, etc. These games are particularly appealing to younger players, "speed-softers", or CoD affectionados

, so only no big surprise there: the younger lads are darting forward and the rest moves at their own pace.
When you go to a site which favours more tactical gamestyle, running will be an option mostly when things go South and your find yourself alone and exposed to enemy fire.
Anyway, don't worry, there is a play style for everyone: from suicidal runners, to stealthy snipers. Airsoft is about having fun, not getting a stroke.
Nonetheless, I remember that after my first day of airsofting, I could barely walk and when climbing stairs I felt pain in muscles I did not even know I have.

But that was only after that fist time.