Certain breeds of dogs (and mixes such as are common in competitive sledding) are incredibly capable. Male Samoyeds (weighing around 60 pounds) have pulled over 2000 pounds in competition, for a limited distance - not sledding. Gidget's mother (43 pounds) has pulled as much as 840 pounds in weight-pull competitions. 20x bodyweight is common for fit Samoyeds (for a limited distance, not sledding). Gidget tried novice weight-pull when she was younger and managed a few hundred pounds before calling it a day.

Gidget's father runs lead (the one yapping in the pic below) and her mother runs wheel on the breeder's 4-dog sled teams. The races they run are one mile per dog on the team.

They are never happier than when they are pulling. Gidget's been pulling me on scooters (20" wheels) and mountain bikes for the past six years. This past winter was her strongest yet. I don't let her pull if it's over 55 degrees.

As soon as she sees the harness coming her way at home, she goes to the front door and waits for The Igidgerod to commence -- usually on the National Mall. Here's a video of a typical Gidg-start when a traffic light turns. Drivers, start your engines....

http://www.viddler.com/explore/Gidgmom/videos/3/

In that video, she was already 4 miles into a 6-mile run. She likes to run so close to light poles that I have considered body armor in addition to my helmet (I concentrate intently so as not to wish for a full-face helmet). She tops out around 25mph, and can maintain 8-10mph for miles, with water breaks. We typically go for 4 to 6 mile runs (well, she runs).