TYpical broodie they lose muscle over their topline. It's not a case of some magical solution and POOF she has a fantastic topline. Its the same case as with any horse thats been out of work for some time, working forward and low into the bridle allowing her to stretch that back. Walking up and down hills is excellent, they HAVE to use their back to do that. Give her a solid month of just walking, walking absolutely everything, then introduce some gradual trotting, again nice and low, forward and into the bridle. There is absolutely no use in riding her with the intention of building topline if you're just going to let her poodle around, so make sure those hind legs are working hard.