The Marathon Course includes Genken Dori (between JAEA/J-PARC Main Entrance Gate and point beyond JR Tokai Station) and Rt .245 (going south).
Expected time period of heavy traffic jam is between 11:30 and 12:30, as there will be a traffic control/blockage.
For the route map, see here.
It will not be postponed even if it rains.
Please be aware that you could be caught in the traffic jam. For example, if you plan to take an airport bus, the bus terminal is inside the marathon route. You should make sure you can reach the bus stop well before the bus departs. You may argue that the bus to airport would also delay to reach the bus stop. However the expected delay of the bus arrival to the bus stop is going to be small (if any) because of the bus route.