When I started training for my very first half marathon, back in late 2010, after some time, I started having very bad pain in my knee. It was mainly my left knee, but sometimes my right would hurt too. It was no fun. When your knees are in that much pain, you can't run, you can't train. I went to my Doctor, and she gave me some ant-inflammatory meds and some exercises to do at home (or in the gym). That's when I started getting serious about my training and needed to include some x-training. it helped so much, I was able to run without my brace and enjoy running...pain free!
After reading this article from Runners World, I realized that this injury is more common for women than men, just because of how we're made (and we place more stress on the knee cap). My problem was that my quads weren't strong enough. See, the knee caps role is to provides protection to the front of the knee to any direct trauma. the cartilage beneath the knee cap has the thickest in the entire body. This cartilage doesn't have any nerve supply, it can become very vulnerable if damaged.