UPDATE:

So I finally did a metric century, with no training other than my 12-19 mile round trip commuting.
Just wanted to see if I could do it based on my commuting only and no training rides.
Undulating terrain that got hillier during the last 30 miles. 90+ degrees and humid. Last Saturday.
It was a nice ride. Energetic, but not killing myself.
Went with a friend from church.
He does long rides, and a century back in May and I asked to go along if he was doing any long rides anytime.
He said he was doing a 60 miler Saturday. So I went along, and added 2 more miles + to make a 100km ride.
It was a blast! No problem. I still felt pretty fresh after 30 miles.
As 40 and 50 ticked off and I still felt strong, I figured it looked like I was going to make it.
So I had a great time and look forward to many more.
Only problem I had was after 30 miles, when the hills got bigger, my head would pound as I went up hills. I think it was from the heat. On flats and downhills I was fine, though. No head pounding.
Thanks for the encouragement and advice, everyone!!