I don’t know how and when God may use me and for what. I want to always be ready. Until my old age.

To be ready mentally and spiritually, obviously, but if God calls me to do something and I am not physically healthy nor fit for it, then I cannot go. So I want to be ready physically too.

Thus I want to maintain good health and exercise regularly to keep myself fit. Until my last days. So even when I am old, I will still be ready to be used.

See
A process steadily maintained