Perhaps some others here are familiar with the issue and have a perspective too. The breed officially is not a sporting breed, it is a toy/companion breed and hence a slightly different role. All cavaliers are different though! and some may be OK for some short distance running -- but still cannot think cavaliers are the best choice for running generally, as the breed does have a higher likelihood than many others for joint problems like luxating patella (common in all toy breeds) and hip dysplasia. And their soft palate and sinuses are affected by the fact that their nose is so short, meaning they have a harder time breathing and therefore, cooling themselves down by panting or in some cases, getting enough air during hard exertion -- it is why so many snore and they commonly get the 'cavalier snort' -- and some can have a quite serious airway obstruction problem. A distance of 5-6 miles seems really long to me for a cavalier to be running constantly; maybe that's mostly because I would find that hard to run myself. .
I had a look as I know this issue has come up in the past, and back in 2007 I posted this for someone with a similar question:
This should be helpful! :
Directly related to your question, see below (note cavaliers are short nosed breeds so that is one caution, as is their shorter legs on many dogs, and the fact that a given dog could be prone to either hip dysplasia or patella problems and long distance running especially on pavement could exacerbate these. So a full vet checkup is a good idea before starting jogging or any exercise regime. The other is that usually as with people you need to work up to any distance very slowly, over time, to develop stamina -- I'd think many weeks to go from one mile to even three would be needed):
Humans are better suited to jogging or running for long periods nonstop than are canines, who tend to engage in short, intense bursts of running with intermittent stops to sniff around, piddle and absorb the scenery.
Remember, dogs will usually try to keep up with their people just because it is their nature to do so. This can mask fatigue and overshadow signs that the dog is overdoing it. So be vigilant and do not push your dog too hard.
Keep your dog's breed in mind when planning your exercise routine. Small dogs with short legs usually don't need to ... or should not ...be walked or jogged as long as larger dogs.
Breeds with short noses may have trouble breathing when exercised vigorously. Short-snouters range from little pugs to bulldogs to boxers and many others.
And don't assume that racing breeds such as Greyhounds and whippets can run marathons. While they are built to run, they were not breed to run for long distances.
And for young pups and big breeds of any age, sustained jogging or running is too hard on their joints.