It is only natural that a fit dog will need more exercise to maintain fitness, no different than a human, but it is a part of dog ownership, especially a breed like GSDs. Keeping a dog fit is called responsible ownership.
Deliberately withholding exercise to keep a dog unfit is no different that feeding a dog a poor diet, both are irresponsible and neglectful.
The best way to determine a puppy's exercise needs is to let them set the pace with plenty of rest stops. Natural surfaces are preferred and manmade surfaces should be avoided.
If you need equipment to maintain control of your dog, understand you’re hanging on to your dog’s body because you’ve lost his mind!