They will test you as they grow older.
This, I think, is what I'm afraid of. My little dogs were always very pliant and carefree. They were spunky as puppies but I never received "challenges" and such that my friend told me she experienced with her GSD. I think I just get nervous that I'll "fail" the dog in some way by not being "enough" of a trainer (if that even makes sense!).
Exercise would not be too much trouble, I don't believe. Where I am moving to has access to two off-leash dog parks, a local all-breed obedience club (despite knowing what to do, I would be attending obedience classes as both a refresher and place to socialize), and it is close to several hiking trails. My wife enjoys hiking and I'm sure would have no trouble taking the pup hiking (if necessary I could even hop in my wheelchair to take the dog along the trails, albeit slowly!).
GSD's carry some baggage that you don't need in a service dog, not saying that can't do it.
Could you please elaborate on what you mean by "baggage"? I apologize if it's obvious and I just missed it! :]