
As JG has said walking should be restricted until your pup has reached maturity. The normal rule of thumb is to give your dog 5 mins exercise for every month of it's age - hence JG said 35 minutes.
I used this rule with my GR until he was around 15 months old. He is almost 5 years old nowand gets between 5-8 miles a day mainly all off lead and does agility twice a week. Our terrier started to accompany us on longer walks when he was around a year old. I don't do a lot of road walking with my dogs, maybe once a fortnight, as my terrier stays on a long line and I tend to walk in fields and woods etc where he can rush around to his heart's content at the end of his line - it's about 40 odd feet long so fields are great for staying tangle free with the line. If I am doing a woods walk I walk further as he is on a shorter lead, so compensate for the lack of rushing around by covering a greater distance.
Being careful with the exercise when they are young does help to ensure you are not putting too much stress on young bones and joints. My GR was a rescue and in a breed where hip and elbow problems are not unknown I erred on the side of caution and can now walk all day with him without a problem.