Held annually since 1923 which is the oldest active endurance race for sports cars?

Answer 1

24 Hours of Le Mans
24 Hours of Le Mans is run over a distance over six times longer than the Indianapolis 500, approximately 18 times longer than a Formula One Grand Prix. Sebring was first run in 1952; Daytona in 1966; Indianapolis 500 was first run in 1911 but is for specially built racing cars, not sports cars.