Strong Will, Weak Body?

Over the past few weeks, I've had some ups and downs. I came down with a chest cold, so I took it easy for about a week and a half while it cleared up, so I wouldn't make it any worse. As I recovered, I got back into the groove for about a week, starting with some shorter runs and walks, then building back up to the distances I was running before I got sick. All was going great, till last Tuesday. I went on a long run of about 6.5 miles. The weather was beautiful, and I felt great, but then, my left knee started to freak out on me. I've never had any major knee problems, and almost never experienced any significant knee pain before. The weird thing is, is that my knee didn't hurt as much while I was running, as it did when I slowed down to walk. It really got bad after resting for a while. It got better after that first day, but it hasn't completely disappeared. It doesn't hurt to put my weight on it, it just hurts when I extend my leg. I've borrowed a couple of knee braces from my sister, and they do help, but I'm afraid I could make it worse by pushing too hard. What worries me more, is my half marathon is than two months away! I'm afraid of losing ground in my training, but then, if I push myself too hard too soon, it may get bad enough to not be able to race at all... Ugh, why is this happening now?