There is definitely a higher risk you inhale enough virus progeny indoors than outdoors, yes. We know from the flu that indoors the virus can stay suspended for many hours. Regular talking can project enough virus from an infected person for the air to become sufficiently contaminated, though shedding varies by individual. The more (longer) you breathe that air, the greater your chance of becoming infected. This might also explain why indoors is a greater risk than outdoors, you spend less time in the air that's contaminated, in addition to the greater diffusion/dispersion outdoors, of course.

Are you saying I could breathe in some, conveying a small amount of the virus, and not get infected? That seems to be the opposite of what highly contagious means.

Disclaimer: Not a virologist.

A virus has to achieve viral entry: it has to find a suitable host cell, and introduce its viral material.

https://en.wikipedia.org/wiki/Viral_entry

Whether an aerosol hits your face, or you breathe air with suspended particles containing the virus, the virus has several challenges it needs to overcome (described in the link above). Its success is not guaranteed. An increase in virus particles improves its chance while a decrease improves your chance.

In theory, a single virus particle could achieve its goal.

I biked in an N95 in Vancouver during forest fire season in 2018. Not fun, but doable and it definitely helped -- without the mask I felt absolutely awful.

That's for a different situation, though. The smoke particulates are very small and are omnipresent, so a mask definitely helps. Coronavirus, by contrast, is only airborne for a short while, so unless you're close to someone who's infected you're not going to be breathing it in anyway. That's why they're saying the masks have limited effectiveness outside.

Oh, when the GP said it's unrealistic to bike in a mask, I thought they meant it would be too difficult. Maybe they just meant unnecessary, in which case I agree.

Yeah, I'm thinking that's what they must mean. An N95 mask doesn't cut down your breathing that much, and you can simply down-moderate your effort a little bit so that you never hit a pant. It's no gas mask.

When you say it's only airborne for a short while, what exact time range are you speaking about?

Useless or pointless?

Pointless waste of a perfectly good mask.