There are a lot of great trails in NJ, but I run in South Mountain Reserve, about 2,000 acres of trails in north central jersey. It's convenient, with well kept trails, and very empty most of the time. I start in Millburn at the Locust Grove entrance near the train station. There's a parking lot inside the reservation. Don't park in the train station lot, unless you want a ticket.
Have a good run.
Douglas L. Darnell