50 lbs isn't a whole lot by horse standards- if you just feed her according to your feed bag's instructions (using her ideal weight, not current weight) she'll probably come along just fine.
When I first got my boy in March, he had been on overgrazed winter pasture with a little bit of All Stock feed and needed at least 50 lbs. Within a few weeks of getting a decent amount of hay (~20 lb/day), rice bran (~1 1/2 lb/day), and a multi-vitamin (later switched to a ration balancer), he was noticeably improved, even though I was riding him 3-4 times per week. He's still filling out in muscle now, but looks like a completely different horse.
It's amazing what a difference a good, regular diet will do for a horse