Toneing my thoroughbreds muscles?

gradual increase of exercise, lunging will get your tb in good shape. I also have a tb and she was sortof underweight when i got her maybe by 100 lbs or so, and by the end of the summer she was very toned and looked beautiful! she had a good diet and i lunged her before i rode her, but trotting also is good because you cna slow down the trot and make them collect and also you can increase the tempo to make them extend. fitness wont happen overnight but it seems to come prettty quickly in horses. just make sure when you start working your horse not to overdo it to quick. i hope i helped