Ah, you're in luck.

The 100 mile course is a figure-8. The first lobe is about 60 miles. Then you pass the start/finish area, and continue on for the next 40 miles, which is the actual 40-mile ride route that goes around Sutter Buttes.

So, once you have 60 miles done, you have the option of continuing on and around the Buttes for the full 100, or calling it a day. It's a really good first century ride in that regard.

The first time I rode it, I did the 40 mile route thinking I wasn't ready for a full century, but if I had known ahead of time the way the 100 mi. course was laid out, I would have ridden the first 60 miles of that instead.

The only downside is that you might get some headwinds the last 20 miles or so if you do the loop around the Buttes (but that'll happen if you ride the 40 or the 100)