Knee pain

I've recently begun running several times a week. During a 5-mile run the other day, I began experiencing pain in my left knee. Specifically, it feels a bit like a bad bruise below the kneecap on the inside of the leg, and the pain has continued for several days - especially when sitting for long periods of time, or even while walking. I want to keep running as I'm training for my first half marathon, but I don't want to further damage the knee if running will make the problem worse.

Has anyone had this experience? Am I ok to keep running through the pain? Are there treatments (brace, exercises, etc.) I can try to reduce or eliminate the problem?