I land in Beijing at 8pm and have a ten hour lay over before flying to Bali.

Katie, I'm afraid your wish may not come true although cab is easy to take. The accessible walls - Badaling and Mutianyu are close at 19:00 and 17:00 respectively. So you won't catch it. Maybe you can change your flight schedule. Or, you may need to expect the next chance. The during the layover go to downtown to explore the Tiananmen Square and Wangfujing Street.