[truculation]

>The sense we have that someone is fit generally comes from their posture.

Exceptions I've noted are joggers with hunched shoulders, splayed feet, leaping too high off the ground, and so on. By no means uncommon.

>Does this idea-retraining work as well as the gym?

Not a gym user but I gained a cm in height within a week of encountering the Alexander Technique (AT). My guess is that the best gym users have good posture too, if only for safety's sake.

It's not so much about fitness and muscular bulk but about whether you are receptive to the proprioceptive feedback from those muscles and joints. AT has a unique way of pointing you towards these sensations but it relies on personal contact with a teacher.