How long can the sled dogs run for twenty mph without tiring?

It greatly varies from dog to dog and what type of sledding they where bred for. Sprinters, dogs competing in short races against time, are bred to be fast, but they often don't have much stamina, and are often greyhound, pointer, lab, and small husky mixes.

Frieght dogs have a lot of stamina, but they are usually bigger and tend to be slower as they are often mixes of GSD, Malamutes, and large huskies.

Then you have distance runners, like Iditarod dogs, who are bred to keep up a steady pace at a long periods of times. Dominated by Seppalla's and 'Alaskan huskies' (which typically have more hound then husky), these dogs can usually run at 20 MPH for at least 30 miles, and well bred ones can keep up the pace for 50 miles or more. However, long-distance running is hard work and often dangerous, so it's rare that they would be running so far without slowing down or even pausing occasionally.