Dogmama is so right, but if you can get some out of the mailman (US) they carry a great dog repellent spray. I don't think it's pepper spray; the letter carriers swear up and down by it. I used to skate cross country ( well cross town) and was harrassed by dogs. A mailman saw it and gave me a can of the stuff. The one time I used it I was impressed. The dog seemed more repulsed than in pain and went away immediately. I was stopped at the time so it could just be an adjunct to stopping.
And Lance Armstrong too!

Miss Liz