FWIW, I do wear a mask outside when I go for a walk at the moment, or at least certainly while I'm on narrow pavements/paths - I'll take it off once I get to a park or somewhere where it's more open and easy to distance though. Probably overkill, but I always put one on before leaving my flat anyway (just in case one of the neighbours is also out in the very narrow entrance bit of my building) so it's just easier to leave it on.
I can understand why runners wouldn't want to wear masks (I tried running in a mask a few times back in April and it was horrible, and obviously the ones with valves in are no good for containing the 'vid), even just walking in one I sometimes feel a bit breathless (though that's probably more just a sign of how hideously unfit I am). I suppose it just feels weird given that by now it's so drilled into us to distance from strangers at all times, I find a bit of an instinctive panic rises if someone goes right by me breathing heavily (especially now that it's cold and you can see people's breath trailing after them), even it's only for a second and the actual risk is negligible.