Cadillac Doubleback:  An unknown poster set the route:

Easy one to beat! "Cadillac Doubleback" Acadia NP cadillac entrance on 233 to kebo mtn trail to north ridge trail to cadillac summit to south ridge trail to route 3 and back. approx 3000' elevation gain. approx 12 mi (I have an old garmin 305, doesn't measure distance with elevation well). Did today with icy conditions on north ridge in 2hrs 37mins.

Jesse Veinotte posted the route:

A friend, Justin Contois, and I are planning on doing a through run of the Midstate Trail in MA from the NH/MA border in Ashburnham, MA to the RI/MA border in Douglas, MA on April 13. I haven't been able to find any data on a single traverse as a run, so we may be going for the OKT. We're shooting for a midnight start on Sunday April 13th and will be fully supported. The trail is nominally 95 miles. [Note that the pair failed to complete the route.]

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.