I finished the couch to 5k a few months ago and carried on running a couple of times a week. However, my left hip flexor started to get quite sore during the run and remained that way for a couple of days afterward. So I decided to stop running and do other forms of exercise which were less repetitive on the joint (horse riding, boxing etc) and the soreness went away.
Anyway I was fortunate enough to so a favour for a colleague from work who runs a physio clinic and he said he would check out the jiont for me. Funnily enoug, it turns out that my left leg is in fact slightly longer than the right - I can see it fairly clearly if I stand in front of the mirror and put my thumbs on the top of the left and right iliac crests. This difference is ony a centimeter or so but it means I spend more tim eon one leg that the other while running (apparently) and this imbalance could be what's causing the extra wear on the left hip (and a little bit on the right knee)
So, he got me to walk across a pressure plate connected to the computer which measures the pressure distribution across the sole of the foot while walking / running and calculates how the foot turns during the movement. Based on this data, they've made some insoles for me which stop my left foot from turning out (pronating?) and the right one from turning in (supinating?). And this has pretty well corrected my posture. I'm only a couple of weeks into the new insoles but I'm going to give it a few more weeks and see what happens. Hopefully they should train my muscles to work more effectively.
The moral is I suppose don't be scared to visit the physio if you have something like this - I got to over 40 without knowing I had a longer leg .....