The route is an out & back of the paved paths of the North Branch Trail system, reaching from northern end of Chicago to the botanic gardens in Skokie. Out & back on the Red trail, and complete loop of the Black trail (36.4 miles total) on this map:

https://map.fpdcc.com/#/?trail=North+Branch+Trail+System

VARIATION:  There is now a Variation which is the same trail one way from Chicago to Northbrook:

The Western Greenway is located almost entirely inside the triangle bounded on the northeast by Concord Avenue in Lexington and Belmont, on the southeast by the MBTA commuter rail and the now-abandoned Mass Central Rail line, and to the west by Lexington Street in Waltham. To be sure, there has been a significant amount of development inside this triangle. But there are also significant swaths of green space.