I recently learned that the Green Lane Reservoir has a nice trail system around it. There's a multi-use trail shared by hikers, bikers, and equestrians, and then there are a bunch of singletrack trails that branch off the main trail and go into the woods.
The singletrack mountain-biking here is pretty good. It's not quite as smooth and fast as Middle Run, but it's a heck of a lot closer to home for me. There are some rocky and rooty sections, but it's mostly relatively smooth and very ridable.
It's a pretty big reservoir and I haven't explored anywhere near all of it, but the trails that I have been riding are accessible by parking in the 3rd parking lot on Knight Road (the one just before Route 29) and taking the trail either north or south from there.