I borrowed a GPS from work and rode my usual lunch loop. It's way shorter than I thought it was:
(It doesn't look like the embedding thing is working. The GPS file is here.)
I was under the impression that it was six miles long, and I was shredding at 14 mph through the woods.
But it turned out to be half that. And I turned out to be going half that fast. Oh well. It's still pretty good mini race course. The first part is a mile long, 500' climb on loose gravel to pavement to gravel. Then the trail heads into the woods. The top section is twisty, and the bottom is rocky, bench cut, and covered in slippery logs.
I've done the lap on my rigid One9 twice, and it's been around 24 minutes both times. So now I have a good baseline.
Once my new cog order comes in, I'm going to give the RIPSS another shot. Then I'll hit the loop a few times on a hardtail. That should help me figure out what's fastest.