Build up to work. Start by light hacking or even trail riding. Then progressively work your way up to building the hind end using transitions and working the topline. Once you've established this, try going over cavalleties or small jumps. But really stress the downward transitions! Hope this is helpful😊
I found hacking great to get my mare fit. We done 20-30 minutes about 3 times a week at a walk or trot along with some days being loosely ran in the arena & a lesson during the weekend. But hacking is what I found best!