http://www.reddit.com/r/elderscrollsonline/comments/2x4ouf/data_mining_mount_racing_and_the_black_horse/

The dataminers strike again! Users @Dominoid and Gladys410 have delved into the depths of the ESO folder and uncovered game files for Mount Racing, a concept first mentioned (And only just mentioned) in the second Road Ahead letter on May 1st, 2014. Dominoid has implied that Mount Racing may be a DLC (Though I personally disagree). Here's what our miners found!

"Greetings, great and glorious participant! The piece of paper you hold in your hand this moment represents an incredible opportunity! Among all the riders of Tamriel, none are equal to the famed Black Horse Couriers, whose daring riders have coursed into battle, bearing the orders of generals and kings far and wide. None are faster, none are tougher!
And now you, dear participant, are invited to test your skill at riding for a chance to join our glorious number and to win a legendary Midnight Destrier, the envy of all horsemen!
The rules for the challenge are as follows:
Clever Talaya has set out enchanted course markers, using a glyph of her own making. You must follow the course markers, passing close by each one, in the order they have been set out. If you miss a marker, you must ride back to it, or risk failing the challenge! No cheating!
Take care, as you ride, however. Enemies and wild beasts will try to knock you off your mount. You will need a mount with great endurance to master the most challenging courses.
To help you, Talaya has given some course markers a special enchantment. If you see two course markers side by side, choose wisely!
Good luck, rider!"

It looks like there are 16 different courses laid out in total - one in each alliance zone and one in Coldharbour. There's an achievement for completing them all and a special mount called the Midnight Destrier which is “bred specially for the Black Horse Couriers, this horse combines speed and toughness for quick movement and resilience in battle.”

Hints to course locations

Here are a bunch of texts retrieved from the datamines hinting at where each course will be set, and in what zone it is set in.

"This course will offer you the opportunity to see many wondrous things. From shining seas to ancient Ayleid ruins, you will see all this island has to offer. Be wary though, as beautiful as the landscape may be, it is also dangerous." - Auridon

"When the first soul-shriven were brought here, the servants of Molag Bal had not yet constructed enough prison cells. Cadwell says that instead they were forced to march—incessantly—down this path. Such suffering…so we ride in their memory." - Coldharbour

"It is more exciting than some silly footrace around the city—I promise you that much. Yes. Talaya has really outdone herself this time. There are only so many places worth visiting in this gods-forsaken land, but you will see all of them." - Eastmarch

""It winds through the forest, following an ancient hoarvoar mating trail. Once a year, the Bosmer used to herd the female hoarvoars down the trail until they were exhausted. After that, the male hoarvoars had an easier time catching them." - Greenshade

"Imperial historians say the occupation of Valenwood was the most ill-advised military decision ever made. The Bosmer ran them in circles—quite literally. They created a devious path through the forest—and that is where are our course runs today." - Grahtwood

"It runs right through the Wood Orcs' territories. They like to hunt, but today, it's their turn to be hunted.You can charge into their camps and attack, or you can try to ride through their ranks. Either way, you'll scare the Oblivion out of them." - Malabal Tor

"It follows an old Baandari trading path that is no longer safe enough for travel. In some places, the forest seeks to reclaim the cleared spaces. Other parts have been ravaged by the Colovian invaders. It will be a wild ride." - Reaper's March

"It will require your horse to be sure-footed and your mind to be sharp. Like a snake, it slithers through these rocky hills and it will bite you if you are not careful.I'm afraid you will not see many friendly faces on this route, wayfarer." - The Rift

"Aha! This one is dear to my heart. It runs past my childhood home, over dunes and crags.It recalls how the galloping steeds of the Ra Gada charged across the sands in their conquest of Hammerfell!" - Alik'r Desert

"The Black Horse Couriers have commissioned a very special course here. We are trying to discover a route that will more effectively warn High Rock of Reach attacks. By running this course, you will be actively helping us defend Bangkorai!" - Bangkorai

"It took some persuading, but the Tribunal gave the Black Horse Couriers their blessing to set our course across their lands. The fertile hills and gorges of Deshaan are truly scenic—not to mention the great city of Mournhold!" - Deshaan

"Dark magic is at work in Rivenspire. Vampires. Daedric influence. Families torn apart. A rider of the Black Horse Couriers will need to navigate these trials and more in the course of our great work. Are you ready?" - Rivenspire

"The Shadowfen is a hard place for a mount. When the 17th Imperial Cavalry tried to subdue these marshes, the Argonians waited in the marsh and struck from among the vines.This course mimics the Imperials' frantic retreat." - Shadowfen

"They say that the Akaviri invaders had never seen a horse when they set foot on Stonefalls. What a terrifying sight that must have been! This course follows the path of several riders who set forth to announce the miracle of the Akaviri defeat." - Stonefalls

Hopefully we will see Mount Racing in the near future. As someone who enjoys just riding around Tamriel and watching ESO's beautifully crafted scenery go by, I greatly anticipate this!