I would tend to agree with Kate here, but you really need to take in the abilities of your own dogs first and foremost.
Some dogs with SM would definitely be uncomfortable on a long walk, while others would be fine. If I had a severely affected dog, I would certainly not be taking it on extremely long walks, Especially not if the weather were really warm ( as noted surely not a problem at this time of year in Scotland!).
Also, some dogs may have more difficulty breathing, or just and may not be very fit and you would in the case of the former not want to do these kind of long walks ever, while in the case of the latter, you would need to work up in much smaller increments than just throwing a dog in to do three or 6 miles (just as you probably couldn't do long distances when you first started out).
But all that said, I think people seriously underestimate what dogs are capable of doing. As one well-known trainer said in a seminar I attended a few years ago, most dogs could travel five times the distance we do in the same amount of time because we have them shuffling along at a slow human speed–I am sure you have noticed that if you are walking dogs off lead they tend to go at a much faster trot and keep returning to where we slowpoke humans are making our own way.
I do a lot of very long walks with my dogs up in the mountains and when I go on holidays with them to cottages. I would typically have Leo and Jasper walking three, six, even 12 miles with no problems at all. Leo, of course, has SM, but he actually is at his happiest and in his best form when he is out on a walk and off lead.
This is why, on a separate issue, I think just taking a dog on a half-hour walk daily is really not enough activity or exercise. People need to do more–which can of course be focused play or mind challenging activities like obedience practice or dog puzzles which equally wear out a dog. A tired dog is a well behaved dog and I think if dogs were given far more exercise and far more activity, there would be far fewer behavior and training problems out there!
Even after a 12 mile walk in Wales which exhausted my partner and me, the dogs basically just needed an hour or two of sleep and they were ready for more activity.