A very fascinating mix and breed (of Siberian & Alaskan Huskies) greeted my husband & I at the summer camp in Jueanu. Siberian huskies are the most commonly recognized sled dog; their coats get pretty heavy. There we learned they do various forms of basic training and running, and they start serious training for the Iditarod in the fall with their owners. (The Iditarod race is run every March, when it’s still plenty cold outside.) Sled dogs can get broken in at 1-year-old, but they don’t often get put in the bigger races like the Iditarod until they’re at least 3 years old.
In the lineup, you have lead dogs, who are chosen to find the trail the rest of the team is to run on. Swing dogs are the second set of dogs, & they are positioned directly behind the leaders. Last, but not least, are the wheel dogs; they are the closest ones to the cart. There are usually 16 dogs in a team that runs the Iditarod.
To keep them warmer than just their heavy fur will, sled dog owners will often outfit the pups in body-length jackets and booties. (They go through an insane amount of booties during the Iditarod race too!) I still think it would be awfully cold; more like freezing to do that race, so it takes a special and committed individual to do it!