Maybe I'm not the person to answer this. But when I was working the the medical field we always had people do the things that they could do, even with difficulty, as long as it was remotely possible for them to do. Because the longer a person is allowed to do a task, the longer they will be able to do a task. I don't see how this line of thinking would be any differant with a dog or any animal. So as long as your dog seems to tolerate walking, I would say keep him/her walking.
In school we were studying the effect of exercise and aging and people who exercised daily were less likely to sustain injuries in daily life, it kept them from being as suseptible to dieseses, and increased health in many ways. I would think the same would also go for dogs.
i would say that that long of a walk is fine. if your dog doesn't seem to be having any difficulties while walking there shouldn't be a problem. once he starts having problems though, i would shorten the walk.....but for now i don't see why there would be a problem walking for that long.