It appears that you are in good physical shape, and from those photos I would have never guessed your age.
Keeping in shape afloat is easy - just live on the boat. Keep in mind that a boat, by it's very nature is a great physical fitness device. This is because the boat is constantly in motion, even when tied to the dock. You body is constantly compensating for that motion, even at night while asleep in the Vee berth. You just don't realize this is happening.
When I traveled south down the ICW and offshore, I consumed a lot of calories, but during trip, I went from 225 pounds down to 195 in just 2 months. During the following three months, while my boat was tied to a bulkhead in Marathon City Marina, where there is almost no wave action to speak of, my weight drifted down to 177. I was still drinking booze, eating fattening foods, playing music and loving life.
So, even though you are unaware this is happening, it happens just the same. In my travels I meet a lot of cruisers, folks that have been cruising up and down the east coast for many, many years. Not a single one of them are out of shape. Unfortunately, I have since become a landlubber and gained back a lot of the weight I lost, but at this stage of life, 77-years-old, I no longer care about keeping in shape. I'm having a difficult time just making it to the next week.
As stated in the onset of my post, you appear to be in excellent physical condition, therefore, whatever you are doing, just keep doing it - it seems to be working!
All the best,