After the Mexican-American War in 1848, the United States needed a way to communicate with its new far western territory. The country had just taken away Alta California and New Mexico from the Mexican government. To link these new lands with the Federal government, overland mail service was established in 1847 (there was a very real threat that power-hungry American commanders would try to create a new country in California). Only a year later the U.S. Mail service began to regularly transport mail by steamship through Panama (no canal yet) to California. John Butterfield, a shrewd businessman with large Federal subsidies, established a major stagecoach service called the Butterfield Overland Express in 1858, which traveled from St. Louis to San Francisco that took about twenty days to travel. The Pony Express finally burst onto the scene in 1860 and offered the fastest way to get mail across the country (claiming they could make the trip in 10 days). At the time, the speed of this mail service was unprecedented. The land in between remounts was harsh, but it was not untraveled. The major Indian problems facing the service were in Nevada, where local settlers had decimated local wild life and pinon trees (obviously, to the dismay of Native Americans). The effectiveness of the route was quickly nulled by the creation of the Transcontinental Telegraph in 1861, which offered nearly instantaneous communication. The link connected the East to California by way of Omaha and Salt Lake City. The first transcontinental railroad, completed in 1869, largely eclipsed the need for stagecoaches. Though horses and coaches would still have to be used to make up for gaps in the railway system (Richard White makes a great case for how terrible the Transcontinental system was in Railroaded).

So, if the Pony Express really was a small footnote in the history of western communication and transportation, why does it weigh so heavily on our national psyche? Later Americans fit the Pony Express into a powerful story of national courage, ascendency and destiny. There is no image that romanticizes the "West" more perhaps than a lone handsome rider, being chased by Indians, galloping through the majestic landscape. Show men and women of the of late 1800s played on this imagery and desire for rugged heroes. "The Capture of the Deadwood Mail Coach" and "pony-express riding techniques" were regular features of Buffalo Bill's Wild West and Congress of Rough Riders of the World. These performers, including the likes of Wild Bill Hitckok and the Miller Brothers, were key to the production American West and Pony Express mythology. The idea of an intrepid mail carrier carving his way through the wilderness fit nicely with Frederick Turner's Frontier Thesis, which theorized that "unique" American character was formed by the conquest and harsh nature of the "Frontier." Turner's thesis has been completely discredited by historians, but still weighs heavily on the American mind. The Pony Express and other mail systems were further mythologized by films like Pony Express (1953), Stagecoach (1939), Pony Express Rider (1976), The Deadwood Coach (1929), and Frontier Pony Express (1939). Often, American myth and history are indistinguishable. The Pony express is important not because of the mail it delivered, but because of its position in American legend next to symbols like cowboy hats, six-shooters, tumbleweeds, spurs, sheriffs badges and broken treaties.

SOURCES

Buffalo Bill's Wild West: Celebrity, Memory and Popular History by Joy S. Kasson

"Reflections on the Pony Express" by Martin Ridge

"How the West Got Wild: American Media and Frontier Violence A Roundtable" by Stewart L. Udall, Robert R. Dykstra, Michael A. Bellesiles, Paula Mitchell Marks and Gregory H. Nobles

Railroaded by Richard White

U.S. Postal Service History

I don't have a direct answer to this, nor am I an actual historian. I do feel like I've studied enough to answer this...since no one else has either.

1. This is a good map of what the route looked like. http://www.bing.com/images/search?q=Pony+Express+Map&FORM=RESTAB&adlt=strict#view=detail&id=08EDB058759B3099118EF89F848ADB17414BB96F&selectedIndex=4
2. For importance, it was insanely short-lived. The telegraph came along a year after the PE's birth. Maybe if someone decided to deliver letters via really fast riding a decade earlier, it may have been more important
3. Prior to western expansion, and even during, the land was insanely dangerous. The messengers would typically be alone, having to deal with not only the natural environment but also the Natives. These guys had to be pretty hardcore. They would make these runs in about 10 days during the summer and 12-16 in the winter. Given that it takes about a week to drive from coast to coast, going from Missouri to California in a week is impressive. Reliability though, if I knew I had a letter coming through the Express, I wouldn't be waiting around for it.

sorry if this is formatted weird, I'm new to redditing.