This is the trail that runs along the western (or northwestern) side of the Hutch. It has several road crossings and has some narrow spots. For the most part, it is single-track. I map it as almost exactly 3 miles, essentially the western border of the Hutch. MAP This is ending it at Saxon Woods Golf Course (at which point you pick up the Saxon Woods Trail) and the northern point of the Twin Lakes Trails.
Unfortunately, except at the ends, there’s no way to get on this trail from a place where one can park a car.
The trickiest part is around Pinebrook Boulevard in New Rochelle. When heading north, there is a very steep downhill south of Pinebrook (going up heading south, incl. during Paine-to-Pain) and a technical stretch to the north of Pinebrook before Weaver Street, by which I mean quick turns and ups-and-downs.
It connects Saxon Woods to the north and Twin Lakes to the south.
OCTOBER 2015: Jenipher Quintanilla has a the blog Running the Path Less Traveled (and is the president of the 100 Half-Marathons Club). She posted the following video, which is a good sampling of the Paine to Pain course. A goodly-portion of the Hutch Trail is covered.
WestchesterTrails 
How long (mi or K) is the trail?
A quick-and-dirty view, essentially the western border of the Hutch, gets almost exactly 3 miles. MAP This is ending it at Saxon Woods Golf Course (at which point you pick up the Saxon Woods Trail) and the northern point of the Twin Lakes Trails.
Unfortunately, except at the ends, there’s no way to get on this trail from a place where one can park a car.
what does OCA stand for?
Old Croton Aqueduct.