Especially with big dogs, the length of how long a "good walk" is depends on each pup individually. I would halve the length of the walk that was "too long" to start with, then go a little longer each day, as his and your patience allows. Oh, the sweetness of a 12-week-old Rottie! Big feet and puppy breath - but neither of you want to get in the habit of you carrying him!

A dog book I have says about Irish Wolfhounds, which is another "big dog" - "These pups should not be taken on long walks, as these can damage its joints. Instead, it should be encouraged to run and play at its own chosen pace."