I was lucky to have been able to ride with a woman who worked free range cattle around Crested Butte Colorado. She mostly rode mules as horses just couldn't keep up with the miles and miles she covered every day. She also had some of the best trained heelers who helped in the work. She often covered 30 miles in a day.
She would say that it took several more years to adequately train a mule, but then they were good forever. I know I had trouble keeping up with her. She covered those miles at a brisk trot.
Her name was Barbara East and she is a fantastic artist who does her painting on cigarette rolling papers! http://www.barbaraeast.com/73259/bio/