The challenge to us is to follow this boundary (as best as the roads/tracks allow). This gives a ride of about 150 Km/90 miles, including a bit of gravel along the Brampton Valley.
Here's a link to a Strava route for you to follow.
If you prefer you can break the route near Islip Furnaces and head back via Grafton Underwood.
Here are some routes that we regularly use for club runs. Use the Categories to limit the routes you see. The compass points relate to where the ride goes from the clubhouse. Routes can be downloaded for your cycle computer, either directly as a GPX or via Strava for TCX.