2nd half of VF Marathon. Plus ~1.35km of side trips (possible water stop/route change - decided against, and grocery for Gatorade.) In winter with tights, I'd probably cut across more fields but that would only bring the distance down by another km or less so 22.5 - 23.5 km is looking like what it will take (O' shortest distance is 21.1km.) Note that the by O' shortest-distance I mean without swimming the Schuylkill River & Perkiomen creek twice, climbing up or down impassible cliffs & fences, or running across the 422 freeway 4 times. If you did all that, and survived, you might manage to get all the controls in 19-20km, but then you'd be disqualified ;)
http://www.google.com/search?q=marathon+in+km