I've been wearing a mask any time I'm running in an area where I expect to see other people. It kinda sucks - it's hot, uncomfortable, etc. But I think it's the right thing to do. I'm not at risk personally, but the mask probably reduces the chance that I will spread the disease to someone else (who might then give it to a few more people, and so on).
The masks helps in two situations. First, if I am already infected and contagious but asymptomatic. The mask then makes it a bit less likely that I will spread it to others. Second, if one of the people running past me is contagious. In this case I may catch the disease and spread it to others before I develop symptoms. It is less clear that non-medical masks help prevent you from catching the disease, but my guess would be that masks are still slightly helpful with this.
If you really don't want to wear a mask, drive somewhere that you can run where you won't encounter other people. Simple as that. That's what I've been doing on weekends. In fact, this is a much better way to avoid contributing to the pandemic than wearing a mask is. But if you have to run in a place with people, at least wear a mask.
Two cautionary notes: Since we live in a broken world without enough N-95 masks, runners shouldn't be buying medical masks, since doctors need them like a million times more. Instead we should use a cloth mask - bandana, folded up t-shirt with rubber bands, etc. And second, wearing a mask doesn't mean you can't spread the disease. You still absolutely can, just a bit less. Staying away from people is still the top priority.