Now that I'm one of the old guys, I firmly believe in the old saying, If you don't use it, you lose it. I'm fortunate that my wife likes to exercize too, so I've always got a workout buddy.
Our main workout is bicycling. Probably 4 or 5 days a week on level ground, with heavy head or cross winds, averaging 14/15 mph for 20 miles or so, on road bikes. If we can't ride for some reason, we've got a 3 mile loop we'll use for a quick hike, averaging 15 minute miles.
We're fortunate enough to have a place in the NC mtns, too. When we're there we bike, but for shorter distance. The hills are steep and the riding is tough, but we still try to get in 1 1/2 to 2 hr rides. Again , walking or hiking is our back up. We've got a 4 mile out and back we like to do. We've got friends that live on this route, so our average time depends on how much BS'ing we do on the way.
There's a trailhead for the Appalachian Trail about 3 1/2 mi form our place in the mtns, so we hike that during the summer too. Mainly with out of state friends that come for a visit. That's a great workout too.
We more or less just believe in doing something to stay active and maintaining muscle mass, cardo health and keeping the weight down.
Wishing all of us a happy and healthy new year. And success with your workout program, whatever form it takes.
RB