This
post has lots of good info.
From a runners perspective, the best loop is up the Undermountain Trail to the AT to the Summit of Bear, down the north side, right turn on Paradise Lane and back down Undermountain. You can run almost all of this, except for a couple pretty steep spots on the way up, and then picking your way down the north side of Bear, but the rest is runnable.
Race Brook trail is beautiful, but too steep for most people to run (me included). But once you get to the AT, you could run south over Race, along a great riddge and then all the way through Sages Ravine, and out Paradise Lane and down Undermountain. But then you'd need a car or bike spot to get back to your car.
The Elbow trail (if you can find it) from the back of the Berkshire School on route 41, is pretty runnable all the way to the AT, then you could run most of the way up Everett. But again, unless you want to go out and back, you will need a car spot to get back to your start point if you to down to 41 another way.
If you, like me, don't mind combining some running with some hiking, then there are many many options in these mountains in NW CT and SW Mass.
Good luck!