Peter,
These dogs are of mixed breeds, such as Malamute, Husky, etc. This race (above) is not very long, somewhere under 20 miles around, and while the dogs are fast, they can hold the same speed in longer runs. For example, the dogs used at the Yukon Quest are slower, but stronger since one year they have to travel from Fairbanks, Alaska to White Horse, Canada. The following year the Yukon Quest starts in White Horse, and ends in Fairbanks.
Huskies at the Yukon Quest:
Then there are real fast dogs that are used for "skijoring," which is a sport where a skier is assisted by one to four pull dogs. But these dogs can't take the extreme distances and the cold temperatures encountered during the Yukon Quest.
Skijoring:
Forgot to mention that Europeans actively participate in skijoring, but they use other dogs that are tall, slim, and real fast (don't know the breed). But if very cold outside these dogs, even during short runs, have to wear booties over their paws, and a body suit or blanket.
This one is from the EU as you can see on the flag on her hat: