My mare was an ex broodmare, and when I first had her she looked similar - huge belly, no muscle, all out of proportion. But in time that will all change.
My only piece of advice, based on my own experience, is to remember that her muscles have NEVER been fit, so your bringing into work programme must take things really slowly. At her age, after doing nothing other than slouch around fields, get a big belly, and give birth, she will find it challenging to get fit.
But it's possible! And rewarding. In my limited experience, a broodmare will always look after you, and always take the sensible option.