She does need some weight and some muscling, which will obviously come from work. It is hard to tell from the pictures as she's not standing square. In a couple of them it looks like she toes out a bit in the front. She reminds me of my older tb mare, she has the same butt, whithers and head set. I feed my mare a low starch pelleted feed, free choice hay, and she also gets smart pak senior flex, which has helped her with her arthritis. She looks like a sweet horse and has a kind eye!!
She looks very sweet and I'm glad the chiro was about to come out and adjust her for you. She does look like she needs some extra groceries and light work (just for now) to build up some muscle. I would put her on a nice senior feed, I recommend Nutrena Life Science Senior, It's a very good feed and should help her out. I would also put her on a joint supplement and possibly put a mineral block in the pasture with her. That's always a good idea. With winter just around the corner I would try to put more weight on her and definitely keep her on a high quality hay, even with grazing in the pasture. More ideas to help you out are soaking beet pulp shreds and mixing it in her feed, it's pretty cheap and works wonders and/or mixing rice bran oil in her feed, really any oil with high Omega3s. I hope I helped! I've seen/used all of this stuff and I know it works.