I second much of what NVEAGLE said (welcome aboard, NVEAGLE). I've trained numerous K-9s of various breeds.
At a distance, nothing to loose with a staff or bear spray (it does work, and the volume is greater), or firearm. Shoot to kill, or don't shoot at all.

HOWEVER, once the battle is joined and you have "fed" the dog your arm:
1) Expect and grit your teeth for bone crushing bite pressure. This won't be a nip! I've had them crush a wristwatch through the protective sleeve of a attact training suit.
2) If you have a blade, use it. The belly and throat are within reach if the dog is attached to your arm. Handgun at this range could overpenetrate and shoot you, and I think it's a more dangerous shot.

IMHO if you think you'll have an easy time surviving (without major injury) the attack of a large and determined dog... you haven't been there.