It's not easy to do the same things on a construction site I did 10 years ago, but if I push myself - a bit more every year, I start to get the pace back. I've had back surgery, a double hernia operation, broke both arms doing a swan dive into a cellar hole, etc.
I love what I do so much, it's nearly as much fun as sailing. That and the weather has my boat still covered. I'll get the bug soon. I figure when I can't do my work, I'll 'have' to sail more.
I wish I had your knees. I've not been good to mine and now I may be paying the price. But who knows, you can't really know what will fail and if past use is the reason. However, if I protect them a bit better and use them more, they start to get better,...