Ditto S_B. A good soft running track and at least a 40 foot running arm on a swinging jenny, is very important. I did as D. Mayfield suggested. I tilled in a lot of Bermuda hay and saw dust. This provided a soft running track that was easy on the dogs foot pads.
To keep a dog that liked to run too hard and hit those four corners of the curve. Which could maybe sling the dog and cause it to loose it's footing. I piled up some extra saw dust into those four corners. Before the running began. Kept extra saw dust in those four corners of the circle. Could rake it back in as needed.
If the dog started out to fast. It would hit those thicker four parts of the track and bog down a little. After several turns and hitting those thicker places of saw dust, in those four main corners. Dog would settle down and fast dog trot with intermediate spurts of loping.
I gave a smaller cat mill that had a 28 foot running arm to Mr. Truett. He set it up and Mr. Gainey tried one of his dogs on it. They did not till the ground and build a soft running track. With this smaller running radius, they did not build a running bank on the corners. Results was they ran the pads right off the dog's feet in short order. Mill had been set up in sand. Yikes !!!
I did tell and show Mr. Truett the correct running track I was using and correct dog collar hook up. They used a pulling harness. Cheers