I'm not sure how far south you are, but when I'm down on Long Beach Island I like to take the trip to Wells Mills County Park in Warentown. There is a nice 8 mile loop that can also be 6 miles if you miss a turn (which I always seem to do).
Depending on where you are: Batsto Village and wharton state forest have a good trail system.
Batona Trail is 50+ miles long and runs from wharton state forest to near Little Egg harbor. (bring your tick spray!!)
A bit north, in Monmouth county, Clayton park has some mild hills and rocky sections.
North of that Mercer park has a great selection..I'll run 10+ miles there almost every week and rarely run the same route twice.
Check out www.mtbnj.com for more ideas.. it's a biking site but has a park by park listing with maps and trail conditions etc.