Hiya, the arthritis could have been caused by many things, born with it, someone/thing damaging her bones, bad nutrition etc, don`t suppose you`ll ever know but the important thing is knowing about it, now you can start minimising its effects & help her have a painless as possible life
I`d let her guide you about walks, if she starts getting stiffer when she gets up its too much or if she starts limping cut back. Best do things slow & try build her muscle tone up. See if you can find any doggy pools round your way
Don`t forget the only way the vet can tell is if he xrays her at least ask him whereabouts he thinks the arthritis is & also the wasted muscle.

Sorry I can`t comment on commercial dog foods as I don`t feed them to my lot
Lets know what the vet says & suggests