The Amazing Spider-Man

There are 10 Xtreme Race Challenges in total.

After a few chapters, you will see the first Xtreme Video Challenge indicated on your maps as a blimp. After you complete the Xtreme Video Challenge, another blimp icon will pop-up, this time being the Xtreme Race Challenge.
They both are indicated by a blimp icon, however the challenges alternate.
(In other words: after 1st Xtreme Video Challenge is the 1st Xtreme Race Challenge. Then after the 2nd Xtreme Video Challenge, the 2nd Xtreme Race Challenge, etc.)

As stated by Dom2096, The XTreme Race challenges are easy except the 10th one.
Races 1-9 I usually ended up with an extra 30 seconds or so to spare, the 10th one I barely made.

As a tip, you have two ways of getting from flare to flare (checkpoints), either Web-Swinging or Web-Rushing.
Web-Rushing is better because it is much faster than Web-Swinging, and while holding down the Web-Rush button, time slows (more-so if you level up web-rush) allowing you to pick where you web-rush to more accurately.

Every few chapters, the next sets of Xtreme Challenges will show up. (along with the earlier sets if not previously completed)

Also, being that you can only access a Xtreme Race Challenge after completing that same level Xtreme Video Challenge, it is basically impossible for anyone to get the "A Dash of Spider" achievement before the "Camera Loves You" achievement. So yeah. FYI.

The XTreme Race challenges become available early in the game, and additional challenges unlock as you work your way through the story. You can start XTreme events by finding the "Blimp" icon on your map. After you complete one Video challenge, you'll then be tasked with completing an XTreme Race challenge next. I waited until I cleared the story and then did these, so don't feel pressured to take them on while you're doing your story run.

The XTreme Race challenges are mostly easy, though not quite the cakewalk that the XTreme Video challenges are. You'll need to zip line and swing Spider-Man towards a series of flares before time runs out. The races require you to touch between six and ten flares (the number per race will be show on your HUD).

At the start of each race, you should align yourself to dash to the first marker. After that, just swing and dash to each of the following markers. You probably won't have any issue until the final race, which takes place over / through central park. Stick to the usual plan, but expect that your swing will take you REALLY close to the ground. Be sure to "stick" each marker, and you'll have it in no time.