You can walk up the Wall and then take cable back down. Can buy the ticket at the top of the Wall. The cable car will take you down to the entrance at the back of the hill. So, it depends on where you are start to climb. My suggestion is: Go to Deshengmen Arrow Gate from your hotel (no direct bus and subway. Perhaps you can take taxi). Then find 919 bus back of the arrow gate and take bus to Badaling. Then follow most visitor to climb the Wall till you reach North Tower 8. Then take cable down.