You need to wormer her, I would suggest getting a fecal sample done first to see what kind and how many worms are present. If she is heavily infested killing them all at once can cause others problems.
Hay good quality-not alfalfa-as much as she will eat.
Any commercial feed 10-12% should be fine. They are all made fairly close in vitamin and mineral balances, and start that in small amounts. Small frequent meals are better on her system than one large one.
You shouldn't expect to be able to "see" her improve in less than 1 1/2 weeks. And looking @ her daily will make it seem even longer.
Muscle will come along with although slightly slower than weight gain.
She is to young and weak to ride, but get all your ground work in,teach her to halter, and ground drive. If you take 6 months to do that, all you have to do is add the saddle and you and she should be ready to ride.
I personally don't use or believe in round pens, they are to restricting and keep a horses joints in torque the entire time. If you lunge, periodically walk with her 20-50 feet in a straight line so she can unbend.