Airsoft and paintball games are a pretty good real life FPS equivalent and more importantly it forces you to get physically fit as opposed to sitting on your bum clicking.
I recommend watching some actual airsoft and paintball games on YouTube. They're about as close to the real thing that you'll get.
You know, the rise in popularity of airsoft was probably driven as much by people coming from 3rd person shooter games as it was people wanting a more realistic military force-on-force simulation than paintball.
Nothing can substitute for the real thing, guys.