I agree RicF. I totally agree that we can all get better with practice, but the level that we can reach will be totally dependent on genetics, not just in terms of talent but also if your body can stand the pounding. Some people get injured a lot, others hardly ever pick up injuries. Some of that will be down to being sensible and resting niggles etc but some people are just more resilient. Lots of people say on here "you can get a GFA if you train hard enough" - that just isn't true. And I could train as hard as I like but I'd never reach the level of Jo Pavey or Emma Stepto (similar ages to me).
I had this earlier in the year and cut my running to 3 miles twice a week as much off-road as possible and flat. After about 4-6 weeks of that I built up the distance again and it was fine. Until I stupidly ran 9 miles in my old running shoes (I'd run 9 miles on another occasion in the right shoes and it had been fine). Now it's come back again although not as bad as it was the first time. I've cut my running to twice a week again and have thrown my old shoes away!