Hi George, I'm your age and thinking along the same lines. I did study martial arts for many years (20 years ago) and thinkng of dipping my toes back in the water. As some of the other posters have said, I would avoid BJJ/MMA at our age, it's just brutal on the joints. I was young and fit when I trained in the 90's (before it became so douchey) and it was painful to say the least. I'm leaning towards boxing too. Bag work, conditioning, pad work, shadow boxing is all great exercise. A well run martial arts school could work too. Not everyone who walks through the door is a fit 20-something. If on day one the training involves getting slammed to the ground or sparring for your life with some tatooed thug, find another school!
And another thing. My old sensei said if the school/instructors you talk to claim they have the 'best sytem/school/teachers in the world', find another place fast! Every style/school/system/teacher has something to offer.