In my experience, you bring a mask with you when you hike, and occasionally put it on for maybe twenty seconds out of every five or ten minutes (or even rarer intervals when hiking in the more remote areas like the national parks) when you pass by someone on a trail and can't maintain distance. You hardly ever have to wear it.
And aside from this past weekend, it hasn't cracked 90 in the Bay Area pretty much all summer IIRC, and rarely even gets over 80.
What's strange about now is that the little neighborhood walks that lead down to beaches and other "hikes" like that are completely packed with people at all times, especially on weekends. It's actually to the point where people are parking multiple blocks away and then walking to the trailhead. It didn't used to be that way.
On trails with 90+F weather you have to put your mask on every 30 seconds with how many people there are, it's a mess.
My mental health is not improving when there are a million people all trying to do outdoor activities in the limited space we have nearby.