It would help if we knew the puppies age and if he's had his vaccinations. The general rule of thumb to stick around for young puppies is about 5 minutes of walk per 4 weeks of age in one session. Some puppies are a little more or less energetic than that but its a good rule. You don't start out just getting up and going, you can start by just milling around the parking lot and letting the pup get used to the sights and sounds and smells. He should get used to the idea of being away from the comforts of home.

If this is your first puppy you should be aware that boxers and german shepherds can be prone to joint injuries and torn ligaments. Your dog should not be forced to walk on leash if its too tired and you should not jog or run with your dog on leash until 18 months of age. Off-leash exercise is much better for these breeds as they can control how much they are moving and stop anytime whereas if they twist a wrist on leash you may not know it and force the dog to keep walking. Also you may want to train the pup early on not to do a whole lot of jumping. Obedience training is actually a great way to train a dog AND to provide mental stimulation. A dog with a lot of mental stimulation equals a tired dog who sleeps well and behaves better. Its almost better than physical exercise.