Jaimie, thanks very much, I really appreciate the info you sent!
Violet, I'd definitely recommend vocal training for singers, particularly with the goal of extending your range upwards a bit but, even more importantly, getting resonance into your higher pitched notes. I've had a years' worth of classes, plus years of singing in various formats, and can hit a G above middle C without going falsetto--which really helps, in that I found quite quickly that I can sustain a pitch around the A below middle C (the note around which an average female voice tends to hover) pretty easily and with a reasonable level of resonance. The most important way to avoid straining your voice is just to relax. Imagine the sound is coming out from the top of your head, and that it's easy. And it's sort of true, since the easier you let it be to hit the right notes, the easier it actually becomes. Of course you'll still have to learn how to integrate all the lessons you learn into regular speaking, but that seems to be just a matter of mindful practice.
I was very happy to discover that I can sing in a low alto voice without issue. So nice to know I'll be able to sing in public!