It is my belief that a tired dog is a good dog! I exercise my small dog quite a bit. He is almost two years old and in excellent health. I taught him to play fetch when he was a pup which we play three times a day: once in the morning, once in the afternoon and once in the evening. We play until he is tired and stops running after the ball. He also gets a walk once a day. Consider teaching your dog to play fetch. There should be a youtube tutorial on that.
One thing I learned in puppy class is that you should never take your puppy on extended walks before your pup is at least a year old, because their bones are still growing. If they go on very long walks at a young age, it can negatively impact their hip bone development, because of the repetitive movement involved in walking on a leash.Take your pup for shorter walks only, no more than ten minutes at a time.