Join a Choir.
Sing Tenor, you will learn all sorts of material that you would NEVER EVER use again (unless you like really wierd stuff). It will however MAKE you sing notes that you would never ever dream of singing.
Choral arrangers really like to push the limits of all of the vocal ranges.
I really don't like quite a large percentage of what I sing with my choir, it does however make me sing notes like High A's that I normally would never sing in anything "Folkie"
You can also sing along quietly with the bass section to improve your downward range too.
If you are a real masochist you can try the Alto and Soprano lines too, just sing them an octave lower when you run out of steam.
JohnB