At age 13 I ran in a 100 yards race against another school on their grass track. About 20 yards from the finish (I was winning) there must have been a bump or rise in my lane and when my foot went down on it I got the most almighty jolt through my leg. [I limped over the line and came third :-( ]. I was in severe pain for 2 days after that (and our sports teacher did nothing whatever to help!).
Years later aged 35 I started running again, long distance.
I found that one leg was apparently longer than the other - I got stress fractures, ran lop-ided etc etc. Then I remembered what happened at school. I reckon that my pelvis was tilted with the impact, and being young, the bones hadn't finished knitting together, so I ended up with a tilted pelvis.
To fix this I experimented with sticky-backed felt pads
(Orthopaedic Felt)which I cut and shaped with scissors and put in my running shoes until I had got the legs equal. However that took some getting used to and I ended up with a stress fracture on the longer leg because it wasn't used to being worked so hard. Eventually I got the balance right and ran for years, with occasional 'topping up' of the felt as it gradually got squashed. (Best race was 1hr 15 mins half marathon at age 43.)
The felt is stuck to my insoles and when I buy new running shoes I put the old insoles in with the felt on.
I can recommend an excellent book for running injuries - The Runners Repair Manual by Murray F. Weisenfeld.
I've just started running again at age 63 after a 2 year lay-off and guess what - I have the problem again :-(
However I'm confident that I can fix it with the felt padding.