I think I am doing rather similar with Candy. I cant walk too far so when we are out driving ponies we are leading her along behind and have built up to her doing about half a mile of roadwork, some of it with the ponies trotting slowly which gets her in either a fast stretched out walk or sometimes running. She is on the lead, not tied on, and is watched all the time I cant see why a treadmill would be a lot different and so long as supervised should be safe.