As I exercise my body, my body will repay me with extra years of good health!

Extra years are the smallest part of the return, and the hardest to feel. Nobody experiences a decade added at the far end. You can't spend it in advance.

What you can feel is much nearer. Sleeping through the night. Getting up off the floor after playing with a kid without making the involuntary noise. Carrying four bags of shopping up to a second-floor flat in one trip at seventy, because you're still the sort of person who can.

That's the dividend, and it pays continuously rather than at the end. The extra years are real enough. They're just not the part you'll notice being handed over.