Copied my reply from another similar post
Your pup shouldn't be doing strenuous exercise before the age of 1 year old.
The joints, bones and ligaments are still growing.
Strenuous exercise puts stress on the joints, bones etc.
You won't notice and the pup is happy to let loose. . . .
However. . . .
In later years the dog is likely to suffer from hip displacia and joint problems many years before they would normally be expected to.
All because the owner couldn't wait 3 or 4 months to start running with the dog when a puppy.
Also what need to realise is that the dog will be pulling you, not just running beside you.
That will put even more stress on the developing pup.
Please just wait.