This is something:
http://www.mapmyride.com/route/us/nj/scotch plains/145124658258538355
I think the more useful part is how to get to Watchung, which is less than 8 miles. The route 28 crossing is foggy to me, it's been a long time. But that's about as direct as you can get. You could hit the reservoir and do a loop, go home, and that's maybe a 22 mile loop. I have you up into NP, then into the swamp, and past my house, and then I just started clicking randomly. I don't know how many miles you're looking for.
The other side is tougher, especially for a new road biker. Dunellen is trafficky, so you'll have to find your way there over time. A few too many potholes and so on for my liking there. If you ride early enough, at least the riffraff isn't so prevalent. You could do the Dunellen thing and right up Warrenville, but that's brutal.
Let me know when you're game. I'll ride down and try to figure out some good roads with you.