This is a shorter section of the Boston Mountain Trail that is commonly combined with Boston Mountain Trail - Loop A
for a larger run.
Loop B splits off of Loop A a short distance west of the visitor center parking lot. Look for the "Boston Mountain Trail" sign marking the trailhead. It's suggested to head counter-clockwise to get most of the initial climbing out of the way.
Most of the trail consists of dirt with some small loose rocks (of course if you run in the fall, you'll have a lot of leaves on the path, so you'll have to use some caution with your footing). There are a few small water drainage crossings that have nice placed boulders to use when crossing.
When you get back to Boston Mountain Trail - Loop A
either continue to the right to follow the Boston Mountain Trail - Loop A
or head left to start returning to the trailhead.
If returning, descend to the creek crossing and then in about 25 yards, head left at the Y that will connect you back up the hill towards the trailhead. You'll then head uphill on this short connecting trail and soon see the Loop B cutoff that you took before. From there, it is just a short run back to the beginning of the trail.