In 2001 I was training for a 5 day 500km non-stop adventure race. Disciplines included mountain bike, trekking/running as well as sea kayaking. I live in a cool location.
Winter training included snow shoeing and cross-country skiing.
My dogs (in 2001 a 4yr old 85 lb. GSD and a 2.5 yr. old 40 lb. cattle dog) participated in much of the training.
The cattle dog ran up to 40 km mountain bike and on one occasion walked with me 85 km of very tough seacoast hiking trail in a 20 hour period. The 40kph bike was a tougher test for him.
My mountain biking buddies rated him as a "pro-class" equivalency, except for sections of open road. At 2 he would have killed (possibly literally) himself to stay with the group on "link" sections of road between single tracks. On technical sections he is far, far superior to mountain bikers. The few group rides of this nature my solution was to let him run his guts out, and then force a stop and cradle him until recovered. We both caught up on the single track sections. I also would step right into a brook with him and cradle him while sitting in the water...At 2 his recovery was very quick: massive tongue in such a small dog.
At 7 he is far more conservative.
In 2001 the GSD and cattle dog both covered 50 km backcountry skiing including maybe 2000 vertical metres with me in 12 hours all untracked and were no worse than I was at the end.
I could run for 2 hours on trails with both dogs and they did fine.
I don't run on asphalt, and neither do my dogs.
I did cut my own mountain bike trail designed especially for my GSD, who missed many rides that had faster sections or sections without water. It was a ride with a very difficult ascent (read, very slow biking: advantage : dog) and a highly technical descent (read, very slow, twisty) and two water stops on a 2.5 km loop. Even the big black GSD was faster than all bikers on this loop.
The dogs were in darn good shape. I recall one 8 hour snow shoe walk in which half way through I had two siberian huskies asleep on my snow shoes before an early end to their day, and my cattle dog destroying a stick.
Also, I followed adventure racing protocols for human feeding with my dogs: ie watch set on count down repeat for 30 minutes and snacking whether hungry or not. of course the dogs are always hungry...
I'd point out that I don't get injured from overuse injuries myself, largely because I think pounding miles on asphalt is not sensible...
My cattle dog is now 7 and completely sound. The GSD is now deceased but remained structurally sound to end of his life.
I was careful with both my dogs in puppy stages up to 18 months but in adulthood I exercised them hard but carefully, and they do fine. I think that dogs are capable of really long exercise, in particular the herding breeds. They can also go hard and generate heat, so long as given a chance of full recovery and cooling.
I am dead certain that many with structurally sound dogs underestimate the capacity of their dogs when properly conditioned and measures taken to avoid: 1) heat stress; 2) joint pounding on hard surfaces; 3) fat dogs.
Also, one great exercise I did with the GSD was to kayak with him running on an uninhabited section of river shore. He went in and out of the water, had a blast and was always cool.
rgds andrew may