You ought to just give it (the double metric) a shot.

We (DH and friends) rode the DM and DH's sister Kelly rode the regular century (I think. She may have done the single metric, but I don't think so). We started with her, and then split off in Occidental where we (DM only - not century or metric) we over Coleman Valley Rd. to the coast, up through Monte Rio. At that point, we joined up with Kelly again (century course, and were with her up through Dry Creek). I can't remember where she went at the lunch stop at Lake Sonoma, because neither DH nor I rode with her through the last part of the ride. She may have had a shorter route that didn't take her to the lake.

I think you could shorten it even after doing the whole Coleman loop. It is worth doing the Coleman loop because there's no prettier road anywhere (except descending Old Howell Mtn. Rd. in Napa).

So sign up for the DM and see how your day goes.