PROTECT THE JOINTS

Be sure your puppy gets plenty of exercise for his body and his brain. A German Shepherd puppy under a year old should never run long distance. ... No running up and down stairs, until your puppy is a year old. Remember, your puppy is still a baby and is growing fast. You need to protect its joints, so hopefully you can avoid any future problems like hip displaysia and elbow problems. Exercise/play for 10 minutes every hour or so is better than 2 hours at a time. Natural movement, playtime with you or other dogs and gentle flirt pole is a great activity time. 

And mental exercise will tired up your puppy as well :)

Zeal at 12 weeks having fun