The bus station is not within walking distance. Take subway line5 at Dengshikou station, alight at Yonghe Palace station and change to subway line2 to Jishuitan, and then walk to Deshengmen and take bus919 to Badaling. The subway ride takes RMB2 and the bus ride takes rmb12 by cash or rmb4.8 by transportation card. Remember to take those bus919 equipped with card reader.

But my suggestion is going there by train (Y567 at 0933 and Y575 at 1319), for traffic jam happens a lot on Badaling highway recently. You can take subway line5 and then change to line2 at Yonghe palace, then alight at Xizhimen. After arriving there, you may walk to Beijing north Railway station to take the train. You make the decision.