I heard about the Redmond Watershed from some people we board with so I thought I'd try it out. Was very glad I did.
The trails are very well maintained (not covered in mulch like Redmond Ridge) and well marked. I did run into a few mountain bikes, they were the most considerate bikers I've ever met. I hope the next ones we met are as good!
Total length of the trip that day was just over nine miles over smooth hard pack with a few rocks scattered here and there for good measure.
There are a couple of trails that are closed to horses here. They're gated though so you don't have to worry about getting into trouble accidentally!
Totally recommend the Watershed for a quick close to home ride if you're in the Seattle area.
I've got more pics, video, and a printable map here if you're interested.