my best advice to you would be to carry something that makes a very loud nose - air horn or something of the like. mace doesn't work on dogs the way it works on people. sometimes you can distract a dog with cookies - drop one or two, then throw them a short distance away so you can get out of there.
irresponsible dog owners are one of my biggest pet peeves. i have the people who walk their dogs, leashless, in areas where leashes are required. i used to have a rescued, dog-agressive boxer who i liked to walk - i always kept him on a leash, and i hated it when other peoples' dogs would come bounding up to us - i had to keep my dog pulled up right against me with his head in my hands, and the dumb owners would always say something like, "oh, don't worry, he/she's friendly!' - then i'd have to tell them mine wasn't, and to get their dog the hell away from us before someone got hurt. i would get dirty looks, like i had been doing something wrong!