Sports Pod here!
It seems as if you have found the cure for the specific problem that causes your injuries. Patients that present with the same problem and have found the same cure are always advised the same thing... if you stat to get pain in shoes that have done 500 miles then you should have binned them at 400.
Have a look online for last years model of the same make and buy them that way if expense is the reason why you try to get as much out of your running shoes as you can.
Quick tip, have a look at the sole of you shoes at the heel, if the shoe is designed with a large protrusion of EVA sole at the heel counter then try swapping to one that has zero heel counter, or a vertical back to the shoe. This will shorten the arc that the foot travels in from heel strike to mid foot stance, and it will decrease the speed at which it does this, reducing the forces that can cause some shin splints.
Molly, not going to get into it here, and forefoot running does work for some, but in my professionl experience they are in the minority. The barefoot running fad has finally and thankfully run its course, and we will see less injurys now until the next big thing comes along, 29" shoes perhaps? :wink:. Not good for my bank balance though!