Another thing that really helped me with getting our OTTB moving well was lateral flexation exercises. The serpentines, spirals, circles, and pole/cone work also helped him learn how to 'bring it all in' sort of speak and really work off of his butt. The picture that I gave you was of Noah about a month into his retraining- and he was about where your girl is right now. Moving well but just not using himself as much as he could.
You do have to remember though, that a racehorse is going to have different muscles developed than your average horse. Noah came right off the track with a few months of down time, so that was really important to remember with him.
With your mare, while she definitely isnt racing fit anymore- she's still going to be using those stronger muscles to propel her forwards, and that's where the retraining comes in. You have to teach her to use and build up those other muscles that she most likely doesnt even realize that she has.
With that said, I'd advise in trying to get her to stretch out into the bit and really relax all of her muscles during warm ups. You want her to be searching for contact with your hands and moving forewards well.
That's the kind of work that I actually did with Noah for the first few weeks, just to get him used to the contact and to using different muscles. And then I started introducing small amounts of work in-frame, gradually working him up to look like this.
In that picture, he's using himself correctly (albeit, a little stiff at the poll) versus the stringy, hollow way that he was moving before. That's what you want with your mare.