I suppose that if I lived in New York City or even NYC suburb Rockland County, I might have some advice for you, or might even be interested in a ride in such a place. However, I don't live in such a congested place. Thank goodness.
Looking at the map, Clifford's comment on a cul-de-sac routing seems a good point. I used to ride a certain road DOWN to the lake, just so I could then ride back UP. Climbing training when I didn't have time or whatever for a longer rural ride. One Saturday, leading my informal ride group on a short route, making it up as we went, staying close to our our start location 'cuz bad weather was closing in, I started down that same road, but realized that what was good for one one or two riders was a bad idea for 8. Luckily, I realized that the dead end was a bad idea in time to turn on to a development road/street and avoid the potential congestion or worse as 8 riders tried to turn around on a very narrow road.
M's advice, which I only skimmed, is likely good for wide open spaces such as Alberta or NW Illinois or north central North Carolina, but Rockland County, except for that large stated park on the northwest end, looks to be housing development next to housing development next to housing development. Probably mature housing developments, but all the same, probably crowded with traffic (according to my def'n) and no where can one expect to ride 25 miles uninterrupted, nor even 10 miles.
You start your post mentioning the good riding available in your county. To me, that means you already know the appropriate roads. You already know the local historical sites you want to include. I'd suggest you string together several of your shorter club routes - I assume you have several routes that cover different parts of the county - and then go ride the prospective route before turning 20 or 40 or 100 people or more loose on it.