
The 5 minute rule has no basis in reality. There is no scientific evidence for it, but there is evidence that it could actually be too little - in a study done on young beagle puppies, trotting far, far further than our pet pups ever would was found to improve joint health, not damage it. Too low an exercise amount has the opposite effect.
So, don't worry about it. Obviously don't run marathons but walk to signs of fatigue rather than arbitrary numbers: if pup doesn't want to keep going, or if pup is starting to act up - jumping at you, biting the lead etc then they've probably had enough, physically and/or mentally. Focus on experience rather than distance/time. Walks at this age are about exposure and exploration.