In the Dobbs Ferry/Irvington/Ardsley area, there are several preserves that are decent for running. They can be run standalone or stitched together for a longer run. The highlight of the area is Irvington Woods, home to the O’Hara Nature Center, the Peter K. Oley Trail Network and the Irvington Reservoir. The 400-acre preserve has several miles of trails that are great for running, ranging from easy to more challenging. This trail map has excellent info on suggested loops.
For a longer run, connect to the 76-acre Juhring Nature Preserve in Dobbs Ferry (use Cyrus Field Rd., caution: no shoulder). These are gentler, fairly wide trails located on a former estate. At the southern end, you can use Ashford Ave. to run east to V.E. Macy Park, an open park with a ~1 mile trail. You can also connect here to the S County Trailway (see our route there).
For an even more ambitious, longer run, connect to our Ridge Road Park/Hart’s Brook Park route in the Hartsdale/Scarsdale area, ~2 miles east using Headerdell Rd., which has a decent sidewalk most of the way.