I have to tell you that my heart stopped briefly when I read your post, then started racing and I felt sick to my stomach.
I am the owner of two mastiffs.
One we have had since he was 8 weeks old. The advice I got not only from my breeder and vet, but other mastiff people, is NEVER OVER EXERCISE A MASTIFF!
A 6 mile hike is too much for
any pup, but
especially a mastiff.
These dogs are not meant to go that kind of distance, not even as adults. They are
low exercise dogs.
I know a lot of the dogs on this forum are from the herding group, which exercises more, but you need to think of the masiff as a Sumo wrestler.
Please, please, please gently exercise your baby.
Although Mastiff's are big dogs, they are very delicate.
Yes your pup went the distance, but they are very stoic and will do what you ask them to.
That doesn't mean that all the exercise is good for them.
The first 2 years will be a upward growth for your pup. The next 2 to 4 years he will fill out. My 2 year old is 205 pds, and his breeder thinks he will go to 240. He gets 'gentle' walks and some play time in the yard.
Please check out these mastiff forums that will tell you what you need to know about the breed. A lot of the posters are breeders who have many years of experience with the breed.
http://www.websitetoolbox.com/tool/mb/acslindahttp://www.websitetoolbox.com/tool/mb/mastiffmessageboardhttp://pub25.bravenet.com/forum/2062396443I know of 2 separate mastiff's that were given unlimited exercise in the first 2 years of their life.
One now has a torn crutiate, and the other has severe hip dysplasia.
From "Your purebred puppy"
Providing the proper balance of exercise. Young Mastiffs need enough exercise to keep them lean and healthy, but not so much that their soft growing bones, joints, and ligaments become over-stressed and damaged. Adult Mastiffs need more exercise to keep them in shape, but not in hot or humid weather for fear of overheating. The proper amount of exercise can be difficult to regulate in giant breeds.
Since you have to minimize their exercise, young Mastiffs can be very rambunctious. They will romp with uncoordinated gawkiness all over your house. You need to substitute extra quantities of companionship and supervision. Otherwise, left alone, young Mastiffs become bored and destructive -- and their powerful jaws can literally destroy your living room.
Here is some info from a breeder site:
http://mastiff25.tripod.com/id15.htmlPlease remember that I am just very concerned about your puppy.
Chris