There's lots of route cue sheets for Bergen county on the web -- but I don't know how to sort thru all of them to find out which are low-traffic in the way you're hoping for.

For bike paths (not a bike lane) there's the Saddle River county park. But I wouldn't want to ride very fast on that because I remember a number of bad-visibility curves (going under bridges). Then there's the "river drive" in Palisades Park between Rt 9W and the Hudson River -- but I don't think that gets up to 25 miles.

I think Bergen county has lots of low-traffic streets in residential neighborhoods. So you could make up your own "low traffic" route with lots of turns that "curls" / "snakes" / "winds" inside various neighborhood areas, then find some clever low-traffic ways to connect between some neighborhood areas. You'd learn to savor different styles and periods of residential house architecture, and develop strong leg muscles from so many accelerations after slowing down for so many intersections. (Not sure anybody else would want to ride it with you).

I think the puzzle is finding a mostly low-traffic route which sorta goes somewhere. I'm not sure that really exists, but here's my own best attempt at a compromise between low-traffic and "going somewhere": Bear Mountain the complicated way
(If start from Englewood instead of the GWB, and skip climbing to the top of the mountain, I think it comes close to the 80 miles you mentioned -- though it still includes a few miles on Rt 9W)Hopefully this will inspire somebody to suggest a shorter low-traffic route with fewer turns.

Ken