The Marathon Course includes Genken Dori (between JAEA/J-PARC Main Entrance Gate and a point beyond JR Tokai Station) and Rt .245 (going south).
Heavy traffic jam might be caused from11:30 to 12:30 due to a traffic control/blockage.
For the route map, see here.
It will not be postponed even if it rains.
Please be aware that you might be caught in the traffic jam. If you plan to take an airport bus, you should bear in mind that there are bus terminals inside the marathon route. So you should make sure you can reach the bus stop well before the bus departs. You should also bear in mind that a bus to the airport would arrive at each bus stop later than usual. However the delay would not be so long because there are not so many bus stops inside the marathon route.