The nominal speed of the Pony Express was 60 miles in 6 hours on 6 horses, which over the 1966-mile run should have been about 8 days 5 hours. (60 miles ~ 100 km.) Each horse thus went 20 miles a day, there and back again. Two minutes was allowed for changing horses. Actual end-to-end latency was about 10 days, 12 to 16 days in winter.
How was the Pony Express as a communications network? Throughput was 1 rider in each direction per day carrying a theoretical maximum of 640 half-ounce letters and an average of 56 (but small packages were also carried, so this figure is inflated). The 165 stations were covered by 400 horses, only 2.4 horses per station on average.
The Pony Express operated for only 18 months in 1860-61, charging $5 per half oz (14 g), which would be about $110 today. Although the price was later reduced to $1 ($22 today), the service was a flop, businesswise. However, only one rider was killed and only one mailbag lost.
For comparison, Arabian camels can do 100 miles (161 km) on a good day, though I don't know how long they can keep it up. Bactrian camels, OTOH, can carry up to half a tonne at standard pace, and can do it in 140 F (60 C) temperatures or arctic-like conditions (on the Tibetan plateau) almost as well. Camel endurance races run about 27 miles (44 km) a day and last for 15 days, no camel-changing allowed.
The U.S. Army tried Arabian camels in the American Southwest as a cavalry mount. The experiment was not a success, because a camel's feet are better adapted for the Sahara's soft sand dunes than for the rocks and thorns of Arizona. They kept going lame. The U.S. Civil War, plus a program of road-building, had a lot to do with aborting the experiment too.
There'll never be a Camel Express, but they're hard to beat for hauling large loads overland, and it's no wonder that camels made wheeled vehicles obsolete in their areas of use, until the coming of internal combustion. Camels rule. On the other hand, camels also have nasty dispositions, and they stink.