I built a 80 footer off a truck rear end axle with a little over half set in concrete. Used 3 or 4 inch heavy plumbing pipe, suspended with over head and side cables to stop side to side swaying. Was a center post going up say 15 feet to attaché the wire cables too.Since I worked for Ma Bell could get plenty of wire cable, turn buckles. Use a large metal ring welded to the upright center post. This allowed the main longest support cable to move free and allowed a give and take. Took pressure off your main bearing.

Doing one today, I would buy those smaller made antenna towers. That CB radio guys use to mount their radio antenna too. Would need minim overhead support with no side way play problems. Could use a 2/3 length running arm with a 1/3 counter weight balancing arm.

The ideal running arm length if you have the room is 40 feet. Run the dog on a extra strong, extra wide, padded dog collar with a smaller back up dog collar. I had plenty of saw dust and Bermuda hay tilled into the running track.

To keep the dog from over running the mill till it learned how to run it. I put some piles of saw dust in all four corners of the running circle. Dog would run hard, hit the saw dust pile and bog down. Two or three laps of that, dog would slow down and rate itself. Cheers