I reserve Saturday mornings and I work very fast and expend a lot of energy detailing the cars. It is my aerobic exercise for the week because I'm occupied with work, family, etc. for all other times.
Detailing is my exercise, I work up a good sweat, get fairly winded and don't let up on the accelerator. Even something as mundane as cleaning and detailing the windows can be turned into decent aerobic exercise. Everything is organized and laid out on the cart and on a portable table I setup so I can bang out three cars in under 4 hours. Most of the time I use spray wax as my QD drying agent. Door jams, under the hood and every other part of the vehicle gets attention. The interiors are sometimes neglected if I'm running short on time.
Periodically I hit the paint with a DA, clay and paste wax, etc. Those are all done with precision and intensity, too. My upper body strength and flexibility is pretty decent for my age, too.
This is what works for me too. My priority is more a workout than a clean car. I just don't see why if i'm going to spend the time exercising, I shouldn't have both.