I must share with you one of the tactics my boyfriend tought me, and is used for scaring the dogs. Instead of waiting for the dog to start runing after us, immediately as we see the unleashed dog without owner in the near by, we starts pedalling as fast as we can but not away from the dog, but toward him. The dog usually gets scared, because we are at least as twice as big as him and we show no fear an is in animal instict not to start atacking if he estimate that we are more powerful, so he runs away. I'm not sure it this would work for the black lab, but it's certainly proven for the other «normal» dogs.
I hope you've undrestood what I ment, cause my English is very bad. I've stop learnig it 4 years ago and since then forgot almost everything.
p.s.good luck with the dogs – to all of us
"Life is not measured with the quantity of breaths you take, but with the quantity of moments that took your breath away..."