I feel that in the 24th century, everybody will be in top physical condition. Like in any navy, paramilitary organization, or civilian space program (or whatever the heck Starfleet is supposed to be; see the current debate in the GTD forum), Starfleet duties undoubtedly demand a high degree of physicality and athleticism.
And creating the right appearance for the role is just part of the job when it comes to acting. Look at how much Christian Bale changed for his various roles, going from emaciated to muscular depending on what was needed. On the other hand, look at how Paul Dano gained weight for Love & Mercy because that's what was needed to accurately depict the subject of the biopic.
Kor