I was a definite PE dodger at school, too. Of the lazy beanpole variety, i.e. my mindset was, "I'm a size 8, so why do I need to?". The health benefits and what years of doing nothing might actually be doing under the skin hadn't really occurred/concerned me. Fast forward a few number of years and I started to wonder. The unexpected endorphin and shopping side effects were a bonus.
Did you go a bit hard at your PR then or did you have a niggle in your shin that you've accidentally aggravated? Hope it recovers soon. Rest as much as possible and if you do have to go out (or start to feel better), stick to well spaced, gentle recovery runs and lots of anti-inflammatories, would be my advice.