Here's my Garmin map and details. Lots and lots of photos below.
At the Martin Road Trailhead (which appears to be on Williams Lake Rd to my untrained eye). I was tempted to bring the Pugsley, but this is the kind of ride that I got the Vaya for.
From the trailhead to Mullinix, it's about 6 miles:
On Belsby - the road to the right looked promising, but it was marked in an unofficial looking way as "no access". There was a truck parked on it a little ways down, so I noted it for future exploration.
Lots more pics on Belsby, an up and down ride. Mostly down with some fun descents.
The ride back started out innocently enough with some more up and down climbing. After I got back up to the more open country, the wind picked up and made it a bear of a ride. One of those rides where you just put your head down and keep pedaling. On the way down I was thinking I should've brought someone along; but if I had I think they would've been cursing me all the way back. But I was glad I made it. I was a little disappointed when I figured I wasn't going to make 55 miles. Next time.