Certainly exercise is important for all dogs. And puppies can be wild and crazy in the house if they have no stimulation or means of entertainment. Exercising your dog can promote fitness and both good mental and physical health.
It is important to understand, however, when working with puppies that long walks with young dogs can be harmful. Young bones need time to grow and form correctly. Over exercise can, potentially, do more harm than good in young dogs.
It is important to understand that mental exercise is equally effective and equally important.
Most people dismiss the idea that working with you puppy can be just as effective as taking your dog for a long walk. When dogs work or train, they are working their minds. While this is not terribly physical, working the mind can be tiring and can stimulate young puppies enough to help them be better citizens during down time at home.
If you have ever traveled via airplane, then you likely understand how mental stimulation can be down-right exhausting. For most people traveling is very tiring. Being in a busy airport, trying to find your way, worrying about getting to your gate on time, passing through security, and dealing with parking or airport transportation are all things that we do not do on a daily basis and when we do venture into the airport to take a flight, we are generally very tired at the end of the day.
This is why our puppies are so tired after they visit a new place or go out in public with us. Even if they only went to the bank for a short while and then slept in a restaurant for an hour, the environment is different and they have to think which makes them tired.
Taking the time in your day for training sessions can also be effective. If you are working on a new skill your dog is thinking and often struggling to figure out what you want. His mind is working steadily and while he may or may not be in a familiar place, to him it feels different because there might be a different piece of equipment or a different behavior that you are looking for. Challenging their minds makes them tired. It is not uncommon for puppies to need a nice long nap after a good 15 minute training session.
I don't take my dogs for many walks. My dogs go outside with me and run and play in the yard and we do at home training sessions. I have a fenced in yard, but they rarely stay out long if I am not with them. I keep training sessions relatively short and sometimes try to do a couple of them in a day if needed. I try to break up the sessions with some review of old skills but also with something new to stimulate their minds. If I don't have anything new I want to teach my dog, then I work on something silly like pushing a ball across the floor with their nose. Silly skills often evolve into tricks and they almost always bring a smile to my face which is always good in training sessions. I use the clicker and treats and reduce the size of their morning and evening meals accordingly.
Training builds relationships and helps stimulate a puppies day, making them easier to live with and more likely to quietly rest or chew on a toy or bone.