Here are some interesting statistics on the historic Pony Express.
Took 10 days
200 miles per day
10 - 20 miles apart between relay stations
Average speed 8 mph
A rider named 'Pony Bob' impressed with a 120 mile ride in 8 hours 10 minutes. I assume he changed horses. This works out to 14.7 mph.
Pretty slow going by today's speed of light standard. The telegraph put the pony express out of business after a few years.
I've seen movies that show the pony express riders galloping off, but the 8 mph average tells me they were going at a trot at best. A human can walk 4 mph on flat easy ground, a lot slower when it gets rough. The Kentucky Derby riders run at 38 mph.
I'd be curious to know what time today's endurance riders are doing 20 miles?