North Boundary - 179 km one way. 

The North Boundary country possesses its own unique brand of beauty- a wilderness of broad valleys and distant views that is inhabited by an array of wildlife. The trail is rugged and many large, potentially dangerous creek crossings are not bridged. Not recommended during periods of high water.Some horse traffic. Elevation gain 2688 m and loss 2922 m, from Celestine Road, 
one-way. Maximum elevation 2019 m.

The Southern New England Trunkline Trail runs 22 miles from Douglas State Forest to Franklin State Forest. The trail is one of the longest trails in Massachusetts, running through 6 towns. 
(https://www.mass.gov/locations/southern-new-england-trunkline-trail)

Begin at the Franklin trailhead and end at the CT border, which is just beyond the gate, at the beginning of the trail the leads to the tri-state marker.