You will have fun no matter what you look like. I've done three Atlantis cruises and already booked for the fourth. There are people of all sorts and types on the ship. Remember, it's a crowd of some 2000 men!
I'd say about a quarter (especially those under 30) look really good though not necessarily all buff, another quarter are 35-45 year olds who spend 5 hours a day in a gym to be buff just so the first quarter would hang with them, and the remaining half is the rest of us: bears, cubs, twinks, geeks, dorks, spazzes, ordinary people. It's all good.
Atlantis' staff gets very involved in all activities and makes everyone feel really comfortable.