hot take: I strongly dislike the use of the word "fitness" in the exercise sense and I'm surprised it survived the wave of deprecations of Words That Are Commonly Used But Are Kind Of Insulting If You Think About Them that's happened over the past n years
things aren't just "fit" with no qualifiers. fit *for what*? there are lots of things a person can do that don't require any particular physical attributes. even within high-intensity physical tasks, lifting heavy things and running long distances are things you have to train for separately if you want to be able to do them.
shouldn't we, like, try not to imply some people are generally "unfit" without qualifiers? even for objects without feelings, we usually say "(un)fit for purpose" to hint toward this concept without needing to explain the task, but that'd be a weird-ass thing to say about a person.