Yesterday I went running and hurt my knee. Could it be serious?

I am 75 pounds overweight and just started running - jogging, I should say, as I am very slow. It was only my fourth day, and I have just been running about two and a half blocks. I wear good sneakers with a custom insole that provides good support. I am running on concrete. Yesterday I ran the first block and a half and slowed to a walk, and my right knee started to hurt. I turned and walked home and by the time I got there I was limping. It is joint pain, not the kneecap or tendons; it feels like the whole joint is sore, but not the inside, just the outside, all around. It hasn't swelled at all. It isn't severe, but it's enough that I can't walk far. I iced it when I got home but haven't got a heating pad right now. Today it's still sore, about the same as yesterday. Could I have really injured it, or does it just sound like normal soreness from beginning running? What's best to do?