Race: Swift as the Western Winds - Novigrad (Hearts of Stone)
Location: Upper Mill/Brunwich
Quest Giver: Ofieri Salesman
Other Requirements: Hearts of Stone DLC
To begin this quest speak with the Ofieri Salesman located in Upper Mill, the Hearts of Stone DLC hub in northeastern Novigrad. Here you will find an Ofieri Salesman (pictured above) who you'll need to speak with for this quest. Compliment his horse and you'll get the dialogue options which will lead to this quest.
You'll find that this horse race is no different than any of the other ones. Use my screen shot below if you'd like to see the route you'll be running on your horse. Even with some of the best gear on Roach you'll be unable to sprint for this entire race - so make sure you try to block the person you're racing against when you stop sprinting. What I mean by block is, try to have them collide into the back of Roach to slow them down.
Reward: Ofieri Stock Saddle