Back of knee pain

Patient: HelloI have been running all summer in preparation for the Detroit Marathon on Sunday. I have started to have problems with a dull pain in the back of my knee and I took some time off, rode my bike and stretched and slowly got back to running. But the pain came back again two weeks ago (dull pain to the back of the left knee that makes my knee feel unstable) I went to my general doctor and he had me get x-rays and blood test. The x-ray came back OK and the blood work showed no elevated levels of infection in my body. So no stress fractures or inflammatory muscle or ligaments. What do you thing can be wrong and what are my chances of running on Sunday?Thank youMats