There is very little to see of her leg muscles in the upper legs due to her thick coat. To me, I just see a gangly yearling. Living on full turnout with a good diet is sufficient do develop healthy muscle mass on a young horse. You do not need to be doing any muscle building work in hand unless she is to be shown in hand.
I think you're making it more complicated than it is. Feed her right, turn her out, and she'll be a happy, healthy young horse.