Never given it much thought but the trainer Saleen is taking classes with says that younger pups shouldn't be going for long walks as it stresses growing bones and joints by over working them. I would think that at 13 weeks old a 30 -40 minute walk is a bit much. Saleen goes around the block now at 6 months old but when we first got her it was down the street or drive to the park and let her wander around so it wasn't really a structured walk per say. More like an outting. I was also advised to not allow her to do things like weave poles, jumping off furnature, jumping in and out of the car and such. I didn't really do any real research on the topic, just figured better safe than sorry and not walking a mile a day as a young pup wasn't going to harm her lol.