How fast can a Quarter Horse run flat out?

Yesterday I went on a trail ride with my train and a couple of my friends and we always race in fields and up hills.My mare is 24 years old and she beat 4 other horse all younger than her.The one horse is a 6 year old Thoroughbred,another is a 10 year old Percheron/Friesian cross,another 19 year old Quarter Horse,and a 7 year old Appendix.We were all running flat out.In total we ran over a mile with breaks in between runs the longest run was a little over a quarter of a mile.