I am early 50s and feel pretty good. Yes, I am old, but compared to most Americans, I am still fitter and more physically able to do things than a lot of guys half my age. In the land of the blind, the one-eyed man is king, right?
Sadly, we are a nation of obese, exhausted, over-worked (yet simultaneously lazy) junk-food eaters. By staying super active, running, climbing, biking, swimming, hiking, mountaineering, and doing triathlons, we can stave off a lot of what the rest of the nation is going through...
And it doesn't even matter than I am objectively slow. Yes, middle-aged runners like me are terrible, but compared to the rest of modern humanity, athletes like us are doing pretty well, right?
p.s. Turning 50 is pretty good in other ways too, since you are usually out of the age demographic that worries about money. Those days are over and that is nice... And my kids are at that age now where they seem to be smart and well-adjusted and ready to live their own lives. They are good people and it is easy to imagine them doing well on their own (if/when I keel over).