This article is from the Hiking/Backpacking With Canines, by Terri Watson with numerous contributions by others.
Just like people, dogs must be conditioned for any performance
activity. If your dog is a couch potato, don't expect her to suddenly
be able to carry a full pack on a 10 mile hike. Some dogs will refuse
to carry a pack that's too heavy, but others will injure themselves
trying. Start off with short hikes and light weight. Gradually
increase the length of the hikes and the weight in the pack. Keep an
eye on your dog's movement, be reasonable in your expectations, and
ask your vet for advice when you're unsure. Some of my friends with
working Northern-breed dogs have reported their dogs happily carrying
40-45% of their weight, but these dogs are in top physical condition.