CREATE TABLE table_52591 ("Race" TEXT, "Length" TEXT, "Class" TEXT, "Circuit" TEXT, "Date" TEXT)

What is the length of the First Union six hours at the Glen?
SELECT "Length" FROM table_52591 WHERE "Race" = 'first union six hours at the glen'
SELECT "Length" FROM "table_52591" WHERE "Race" = 'first union six hours at the glen'