I was getting on quite well with C25K, but my right knee was getting a bit painful; I thought it was just not used to exercise, so told it to "Man up, knee!", and soldiered on. Then we went on holiday to Cornwall a couple of weeks ago, and did a walk that was supposed to be 6 miles, but ended up being more like 12, as we got slightly lost (that'll teach us to try and navigate using just a handout from the camp site...) After that, I could barely hobble on a very sore and swollen knee that was making a variety of excruciating grinding and clunking noises.
Long story short, the specialist thinks I've either torn a hole in the cartilage or damaged the back of the kneecap, and given me a natty pair of elbow crutches; I'm to soldier on for a month or two, after which I've to go back if it hasn't improved at all. Lovely. Oh, and told me to lose weight, as well, which is fair enough, but it's one of the reasons I started running in the first place!
Oh, well, you have to laugh, don't you? I'll be back...