I'm sorry to hear about your frustrations. I like the suggestions already presented (except for CesarRunning).
Perhaps it would also be useful to work with dog groups on a campaign to create some off leash areas in the parks — places where dog owners could legally have their pets off leash. We have those in several of the parks in my county and I have not had any negative encounters with dogs in parks. Though I don't run in parks as often as you, I have run in them enough that I would expect to have encountered some problems if things were as bad here as they obviously are there. I cannot imagine that dog owners in this area are simply more responsible than they are where you live. I think that they've provided alternatives and they enforce the rules.
Of course, even in those areas, irresponsible dog owners can ruin the park for others. Owners should still only take their dog off leash if they are well trained with a good recall. When we lived in town, we had a few incidents at the off leash parks with irresponsible pet owners and we stopped going.
The only dogs that I have met who were illegally off leash on the multi-use trails were dogs that would respond promptly to their owner's voice commands. Rather than being threatened, I was impressed with the relationship between dog and dog owner in those cases.