Getting her muscled up will require you to build her abdominal muscles. You do this by lots of long trots on a loose rein up hills and doing lots of trotting caveletti.
Just like with humans.. if you want your back to be strong, you build your abdominal muscles with hip curls and stomach crunches.. and try to get rid of the pot belly and support your back with those abdominal muscles.
With a horse it is the same thing. You build the abdominal muscles and this will naturally RAISE the back and allow it to relax and better carry weight.
It does not matter if the horse is a trail riding horse or a top level jumper or dressage horse. ALL horses are better for getting their ring of muscles working and supporting their backs so they can work more freely and with less chance of injury or pain.
A supple horse is the result.. and that supple horse is more able to work.. no matter the job at hand.