I've tried a few approaches. A surgical mask (which I keep re-using, come what may) and a bandanna. In both cases, I let it slide down when I'm far away from everyone and pull it up whenever someone closes in. I've used it for running, cycling, and walking. (In addition to shopping, etc.)
1) Running with a mask is harder. Something about trapping your breath against your face and re-breathing it
2) Lots of other runners and cyclists are not wearing masks at all
3) If you can run, you probably aren't (very) sick. I know: asymptomatic people can spread the virus. But you'd have to be very asymptomatic.
4) Who knows how long it will be before we can swim again. It's very hard to swim with a surgical mask on!