Whippoorwill Park is in New Castle (Chappaqua), NY close to Greeley High School (see map here). It is 167 acres and has a perimeter loop of around 2.5 miles. There is also an unmarked “purple trail” that is one way, terminating at Whippoorwill Lake Rd. If you add the purple trail to your run your will increase the distance by just about a mile.
The park was originally part of the Berol Estate and was purchased by the town in 1964 and designated as passive parkland. The park entrance and parking area is on the left hand side of Whippoorwill Road 1 mile south of the Route 117 (Bedford Rd) intersection. The trail map is available here. There is also an entrance to the nature trail off Kitchel Road.
I have not run in this park often, but it is very centrally located in Westchester where many loop runners hail from. The trails are a combination of single track and old dirt road beds from the former estate. The hiking trails are well marked with trail signs detailing tree species and other natural highlights on the nature trail. The trail system circles around a hill with the blue and yellow trails reaching the peaks of the hill.