Given the way what we feed our horses changes them, can we expect to see changes in our characters based on our own activity level.
Meaning if I'm a crafter who only travels by horseback will my character start putting on the pounds and maybe suffer stamina de-buffs, or if I'm heavy into combat always on foot running everywhere will my character become super fit gaining stamina buffs

I do not think we can expect to see that kind of realism in an MMO yet. Appearance changes, maybe, if the fans generate enough positive feed back to the idea. I just can't see this going over well with the MMO crowd. The RPG crowd might find it amusing, but I doubt all of them would be aboard the idea. Sorry, did not mean to crush your dreams if I did.

It's not a dream, just an idea. One I think is worth evaluating.

You have no idea how fit you have to be to ride a horse over long distances, do you? Its not a case of sitting there doing nothing while the horse does all the work.