I invest in my health because I am worth it.

Investment is the right word, mostly for the timing. What you do this morning pays out around your seventies, in a currency whose balance you can't check.

The walk today buys a knee that still works at eighty, when you want to get down to the garden and back up again. There's no receipt issued, and no confirmation email.

Which is why it takes something like self-regard to keep going. You're doing a favour for a person you've never met, on the strength of believing they'll turn out to be worth it.