Your dog can take up to 3 years to fully mature but you can start running, tread mill, flirt pole, or other exercises. I feed Kirkland dog food from costco as well and my dogs look great. Genetics play a huge role in this but you can help with exercises. Build muscle tone with a flirt pole that is what I like to use but I like lean look.
This dog is ran on a flirt pole and does agility and schutzhund and is in great shape.
I do use muscle up powder for dogs to add extra definition but only for shows. I do not like keeping them on it for a long period of time. It seems to make my male more aggressive and on edge kinda like roid rage! lol