who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public/myth-busters#exercising

People should NOT wear masks when exercising, as masks may reduce the ability to breathe comfortably.

Sweat can make the mask become wet more quickly which makes it difficult to breathe and promotes the growth of microorganisms. The important preventive measure during exercise is to maintain physical distance of at least one meter from others.