At that age most of the exercise they need will be gained running around the garden. The only real reason for taking them out is from the socialising point of view. A little walk of a couple of hundred yards or so is quite enough. Try to vary the walk. Along to the local shop to see plenty of people, to a busy road to see traffic. (Not to fast and frightening!) Maybe a trip to the local Dog Club to see other dogs. When my young Labrador was that age she spent half an hour at a small exemption show (most of it asleep on my lap!) but it all helps to show your puppy just what the world consists of.
The reason for the restricted walking is because at that age the bones are still very soft. The hip bones (the pelvises) in particular are susceptible to damage. So gradually scale the exercise up over the course of the first year or so.
Regards, John