Having raised 5 puppies, and walking them right about 2 miles by the time they were a couple months old, I've never had any dog with HD or ED. Granted they weren't GSD's but they were medium/large breed dogs with the same general growth pattern. Now, most of the time they were on grass and if we went farther they were off leash in the mountains so no repetitive pounding the pavement.
12 months seems like a really long time to keep a GSD restricted to such limited exercise... I can understand not do miles and miles on pavement when they're under 5-6 months old but after that... personally, I would limit strenuous ball play, jumping, and fast turns/high speeds more than I would worry about limiting leisurely walking, especially if you can find soft ground.
Tango, lab mix 10/01/2000
Scarlett, cattle dog mix 04/01/2007
Sasha, GSD 02/01/2008