How much did she race? This could give you a lot more of an answer regarding the wonder about arthritise.
I would not lunge her, that is hard on the joints if they are inflamed and sore or arthritic.
Long walks, longer warms up, stretching all four legs lighty before riding will help increase the blood low to warm up the area and make it a little eaiser on the joints.
Depending on what the vet says you fan do so many things. For maintiance cause of the harder work for a draft horse (friesain) my trainer goes to costco and grinds up the human MSM pills and gives that to her in grain. I can get the dose amount if that's something your intrested in doing, it's the cheapest pourest way to get MSM that we have found :)