doberman puppy exercise

My lad turns 7 months old this week and I have recently increased the amount of time that we exercise per day in an attempt to curb destructive behaviours that were, evidently, arising through a surplus of energy. I was scared of 'overdoing' the exercise with him at first as I had a vet tell me...

How much exercise is enough for a 9 week old puppy? I just bought a harness for Apollo,so he couldnt run from me when we're out,but now Id like for him to go on walks with me,but I dont want to over do it with him.