Be sure to leave extra early. The bus drivers will know which routes and roads are blocked, but there will be many traffic backups on the open roads. The buses will take you to the TTC where you will have to take the monorail or ferry to the MK. Buses cannot go to the MK since the marathon route goes up world drive past the Contemporary which prevents the buses from turning left into the MK bus stop area. Buses to the TTC also get backed up waiting to get through the traffic light to make the left turn to get to the TTC.
My recommendation is to take a taxi to the Contemporary and walk to the MK. You will have to wait for a break in the runners to cross the street, but this is the best way to get around the traffic jams caused by the marathon.
