This gets us close to 20 with the majority on dirt roads... Either direction would work... Though my place to yours would finish with the downhill on Old Springfield rather than climbing it first thing...
Could also 'cut the corner' from mile 5 to mile 8 by taking trails, and if feeling the desire to trim mileage later could take the Old County Rd (14) to Old Springfield (17)
Earl - if you felt inclined for less mileage you could break off around mile 9 and head down Webster Rd back to my house to pick up your car - probably 12 miles total..