Because I was running a lot I could feel my legs get stronger. My thighs became like steel, and it was great (I love muscles). So even if I did weights or strength training at home, I wouldn't really bother with leg exercises. Why should I? My legs were strong, and I didn't want to overuse them.
This, it appears, was a mistake.
I finally attended a physio about two weeks ago, and it turns out that I have issues with my ITB. ITB syndrome is a very common issue for runners, and in a nutshell, it means that my muscles are both very tight and very weak at the same time. Awesome.
I now have a raft of stretches and exercises to do to both relax the muscles and build them up. In particular, my glutes are very weak so this week there's a focus on them.
The moral of this story people? Cross train. Make sure you're stretching and strengthening everything. Simply running won't just make you magically strong (sad but true...)