I am in great practical shape but need to get into 'gym shape' asap. Tips ?

As a PI, I'm in great practical shape I mean I can chase down any one, lift heavy objecks like full beer kegs and I still can take out any two guys with my bear hands but I have an asignment comeing up where I need to be in gym shape in oerder to 'blend in' . The asigment starts next week . What's a quick way for me to actualy get in good enough shape that it looks like Ive been going to the gym all my life ?