The Bearsden Forest Conservation Area is a beautiful spot for a trail run. It's located in north central Massachusetts in the town of Athol. There are two parking areas that I know of, one on South Royalston Road just after Cobb Hill Road and one at the end of Bearsden Road.
I was able to do 16 miles out here while only repeating a few small sections of trail. You can get some good elevation and there are a variety of trails from single track to wide woods roads, to only knowing where a trail is because of markers on a tree.
You can see by my running route below that there is a large network of trails and you could run this area in a variety of different loops or directions.
The town of Athol has more information on this area Here
I have also made a short video showing some of the trails on my youtube channel Here