The Speed / 10 = MPH works well and is fairly realistic guideline assuming good terrain, roads, or paths. That would result in 48 miles per day. Maybe a bit high, but not unreasonable.
That being said, a typical horse walks about 4 miles per hour. But during an 8-hour day of travel, horses can trot (8-12 mph), canter (12-15 mph) and gallop (25-30 mph) even, increasing the effective distance covered. Trots can be maintained for extended periods given breaks for walking, but canters are more limited, and gallops severely so. The equivalent would be a person walking, jogging, running, and sprinting.
You might walk 3 mph, but could jog for 1 minute in 5 and increase your distance from 24 miles a day to 28 miles without extraordinary effort.
In the same light, a horse trotting (8 mph) 15 minutes and walking (4 mph) the other 45 minutes each hour would cover 40 miles in 8 hours. Horses can easily do this given periods of rest every hour or so. If you "push" the trot (12 mph), the total daily distance is 48 miles in 8 hours (as calculated more simply above).
Over good terrain, I would have a party mounted on horseback move at least 40 miles per day, and maybe 50 max. Anything more than that an you are definitely going to strain the animal.
Hope this helps. Cheers.