That sounds like nail fungus. Consult a doctor if you want to treat it effectively.

https://www.cdc.gov/fungal/nail-infections.html

But would keeping your shoes fungus free keep your toenails fungus free?

Might be worth trying. Might not solve the issue if it's already there but could well help preventing it somewhat, although I don't know what cause this condition though.

I've had fungal infections in the past, between the toes mostly, but it's a thing of the past now. Nothing in the past 5 years.

Have a look at Big Clive's video on "Fixing your feet":

https://www.youtube.com/watch?v=wPLwczrGikM

Probably not. Sounds like toenail fungus. I've had it and it is basically only treatable with oral Lamisil - which is nasty stuff, but does work well.