Wikipedia:

There are two main trails that partly overlap in the Middlesex Fells (https://www.mass.gov/locations/middlesex-fells-reservation).

This is the Reservoir Trail a 6 mile challenge.

Ben Thompson appears to have the FKT, 39 m 39 s, April 2016.

The Wapack Trail in southern NH is maintained by Friends of the Wapack Trail. There is an out & back race on this 21.4-mile trail, and the race website has information on FKTs (splits in the race).  Overall 21.5 (one way) & 43.0 (out & back) mile course records are here:  http://ultrasignup.com/results_event.aspx?did=19292

http://mohawktrailct.org/

The [24-mile] Mohawk trail is a blue-blazed loop starting and ending on the Appalachian Trail in Northwestern Connecticut.  Its southern beginning is in Cornwall Bridge and it winds  through Cornwall to its northern terminus in Falls Village.  Along the way are stunning views, quiet ponds, old country roads, and plenty of exercise.

The Mohawk Trail was originally part of the Appalachian Trail before the AT was rerouted east of the Housatonic River in the 1970s. 

Originally the route was from Rattle River Trail Head as also seen from the early FKTs. Now the trail goes from Gorham and the Carter Moriah TH at Bangor St.

The Dartmouth Outing Club describes the route:

The Carter Traverse is a strenuous 20-mile hike over the Carter-Moriah and Wildcat ranges of the White Mountains, on the east side of Pinkham Notch. It covers six 4,000-foot summits and, despite having no above-tree line areas, provides outstanding views of the Presidentials.