Hi, my arrival in Capital Airport is 12 noon and flight back at 8pm.

Riza, I'm afraid the time is not enough. The walls are located in suburb areas of Beijing. Even though you hire a cab, it still needs around 2h for one way. Your flight departs at 8pm and you should arrive at the airport at least 2h before that time, which means you only have 1~2 hours visiting the Wall. Such a short time is definitely not enough to finish the hiking. Maybe next time.

Thank you for the info...