The trails here have recently been established. They were not built by mountain bikers but are intended to be educational trails. Nevertheless they do ride nicely. Most of the trails are south of the power lines and east of Upper Flannagan Pond. Those are also the easiest to ride. The trails north on the powerlines (Habitat Trail) are more challenging.
You can easily spend 30-45 min in there. If you look for more riding just follow the trail under the power lines to the South Groton trails.