CREATE TABLE table_13503 ("Year" FLOAT, "Competition" TEXT, "Venue" TEXT, "Position" TEXT, "Event" TEXT)

Where was the London Marathon held?
SELECT "Venue" FROM table_13503 WHERE "Event" = 'marathon' AND "Competition" = 'london marathon'
SELECT "Venue" FROM "table_13503" WHERE "Competition" = 'london marathon' AND "Event" = 'marathon'