I have put up a small jump on one side of my round pen, poles on the other, and a pile of dirt on the remaining side (so three "sides" plus the gate) and just sent them around the pen. The combination of poles, the jump and the dirt pile does wonders for their muscles.
Jumping repeatedly can be straining, so usually I will turn her before the jump on each side every once in a while so she goes over the hill/poles twice or three times as much as the jump.
Start slow and work your way up.
I do this in a 60 foot round pen, and had a trainer build/grade my fake hill so it's safe and easier on the joints. I still have the blueprint somewhere if you are interested.
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