In Alaska they exercise their dog teams by hooking them to a 4-wheeler in summer months. The man I was watching had to use the brakes once in a while to slow them and so they pulled harder. They would be running so fast when they went around a corner the 4-wheeler would slide sideways.Although I have done it a time or two many parks heavily frown on tieing anything to trees. Anchors or fences seem to be the best solutions. Our last Lab mix loved to run, so I invested in a Walkie Dog for my bike. Hooked him up and we would go for a bike ride. I clocked him one time at 22 MPH pulling me! I got some negative comments from a few people claiming I was abusing him. I just laughed at them and explained HE loved to run amd that I let him dictate when how fast and how long we rode. Dogs, especially working dogs need exercise to be happy, so the OP needs to figure out how to work his dogs to keep them happy, then he wo t have to worry about them so much.