It was a combination of a slightly lucky first lap and having the AI fast enough that I wasn't running them over in the middle sector. Then I ran a clean enough race to hit a couple of :15s and no :17s. I probably tried a few too many times, but every time I thought I hit the limit I would see a way to go faster in another corner, and every good race seemed to have one or two really bad laps. So I kept wanting more.
I think I have the replay saved but it's not exactly very interesting after the first 2 corners of slicing through the pack.