The wall sits help to not only work your core but also helps to strengthen your quads which can help prevent many injuries including knee pain.
Superman
One common problem in runners, especially those with a weak core is low back pain. It often occurs after a run but can be quite debilitating if it continues.
The superman is a great exercise to help focus on and strengthen the lower back. You and do this move with or without an exercise ball between your legs. This will increase the intensity. Start by lying on your stomach with your arms out in front of you. Engage your core and as you inhale lift your knees, arms and chest off the floor. Then slowly bring your body back down.