Help on feeding my horse ish x tb x id?

i have recently bought a 4 yo ish x id x tb mare she is in quite a bad way she has no muscle definition and has a few nicks on her she has excellent bloodlines and is the granddaughter of king of diamonds she has obviously been bred to jump and is good on the flat whether this is due to her having no weight to her i dont know but i am willing to take the risk so i obviously need to feed her up and get her topline improved i am just wondering what would be the best thing to feed her and the best way excercise regime for her without putting her on anything slightly heating . i have bought her as my first horse after 3 years of owning ponies and am hoping to eventually bring her on for jumping when i have got my confidence on her. any help would be excellent thanks for reading