Hackensack High School's track is open early (6am?) and is open to the public (at least before and after school hours). There is also a track at Overpeck Park in Leonia that is always open to the public.
I believe there's a track behind Bergen Tech (Hackensack Ave.), but I'm not sure about it's availability to the public, and I've never used it. If you are looking for running routes in Hackensack I can give you a few suggestions.
- Carl