I did my first ride ever at lewis morris and I'd say it's pretty much perfect for the beginner rider who wants to improve skills/fitness. Yes, there are climbs, but they really aren't awful unless your in fairly poor shape and either way they are extremely smooth and manageable. I think this is important to note because even dealing with small obstacles with a maxed heart rate will have a beginner off the bike and frustrated. The descents have some good flow and some tight switchback which will improve bike handling without entering "omg I'm gonna get hurt" territory. Patriots path makes a great warm up/cool down. When I first started I would do a loop of "real" single track at lm for 4-6 miles and then do patriots path for another 5-15 miles after to help get my base fitness up, it helped a lot.
Not really north NJ but 6 mile run is fantastic for beginners and worth hitting up if you have some time one day, nice little flowy roller coaster of place with only about 500 feet of elevation change over a 13 mile loop (vs roughly 100ft/mile at lm).
KVSP I've been too once, and thought it was great, although I don't think I would suggest that place to someone very new as the basic loop is roughly 6 miles long, can get somewhat technical, and there are a fair amount of bears in there (I saw 3 the only time I was there) - this place would be good place to come in a couple months when you are feeling more confident
No one mentioned Nassau trails which I think would also be nice - trail layout is a little confusing IMO but it's a small park so even if you get lost it wont be for long. flowy kinda like 6 mile, a touch more climbing, and with some rocks. Would definitely recommend as well.
Basically just keep at it, March 2017 I'd never mountain biked in my life, and quick loop around lewis morris with a guy twice my age whooped my ass. Now I'm completing 10-15 mile rides several times a week over much challenging terrain enjoying it, and even did the race at lewis morris for fun. ( although I still routinely get my ass kicked by guys twice my age!). Hope this helps!