This weekend I experienced a similar situation. I'm 27, have never had any problems of this nature and after 20 miles, it felt like someone was driving a nail into my knee cap. It only hurt on the down stoke so I just took some effort off my left leg during the down stroke. The pain seemed to come and go and vary in intensity from hardly noticable to almost unbareable. I haven't made any changes to my bike in the last couple months. The pain actually started about 2 weeks ago when I got my bike back from the LBS, who only adjust my shifting. I can't imagine them adjusting anything ride related. Could this just be an adjustment problem?