So its a combo of them wrapping their identity into this one thing (running here), eventually not being able to keep doing it, but also there is an aspect that I am purely assuming which is they could have done more variety of stuff with their life. This last point being super biased as its based in my own desires, and who knows, maybe they LOVE this one thing and that's all they want for fulfillment.

But to your point, i also love hearing stories of old people defying our expectations of what an old person should be able to do.