HarbingerX
Rob Of The North
Then you need to look again.
While the carrying capacity apparently is realistic, the effect of going over this limit is not. A horse does not slow down to 10 ft. per 6 seconds when it carries 300 lb.
I don't see how a horse carrying 300 lbs. being limited to 68 mph tactical speed is somehow unrealistic. A check of wikipedia shows that the fastest recorded sprint speed of a horse over a short distance is 55 mph. And you can bet it wasn't carrying 300 lbs. of gear.
If anything, all existing tactical speeds are too high.