For big breeds, it’s especially important to avoid excessive running/exercise before the growth plates are closed. Swimming is fine and a great way to tone. Walks are fine too. The challenge with exercising pups is that so many are breeds that live to please their people. This means that they may push themselves past their threshold just to be with you. So you need to be aware of the signs that they are tiring out, hurting, or otherwise need to stop. A popular slogan in the giant breed world, particularly mastiffs, is: “Never walk your mastiff pup further than you are willing to carry him back.”