Spend all your time working on permanent solutions to the issues of aging, death, and/or political conflict, or whatever other big issues you think are important.

Should they keep their muscles flexed 24/7?

You are using "flexing muscles" as a shorthand for something, and I don't know what. Flexing muscles to impress others is silly; you have better things to do. Flexing muscles to exercise might make some sense, but infinite willpower doesn't imply infinite ability to recuperate from exercise. Stay within the limits of your body so long as you have a use for it.