I like Susan's suggestion about carrying treats. You could become their best friend.

The walking staff is a good idea, too. Useful against two-legged predators as well as four-legged.

If I were encountering those kinds of dogs on a regular basis I would for sure be carrying my bear spray:

http://www.rei.com/product/623173

I do always carry a smaller canister of pepper spray. It's clipped to my dog's water bottle and poop bag holder. I carry it primarily to protect my dog from off-leash dogs. Have never used it but have pulled it out a couple times to make it clear to the owners that they better get their dogs under control.

http://cabelas.com/cabelas/en/templates/...&hasJS=true

According to this article in the Los Angeles Times, mail carriers are given instructions on dealing with dogs as well as a canister of pepper spray.
http://articles.latimes.com/2007/aug/23/local/me-pitbull23