For anyone interested in the bio-mechanics of running this book, by Joe Ellis, a podiatrist and runner, will be very interesting. It's out of print IIRC, but still available at Amazon:
Running Injury Free.
He convinced me that my 9mm difference in leg length is a serious problem for running, or even walking, at any speed. I now wear 9mm lifts all the times on the side of my short leg, externally or internally, depending on the shoe.
Forgot to mention that he also convinced me to determine if there was a difference, as suggested by a physiotherapist who was treating me for a back problem. Not as easy a thing to do as Joe Ellis suggests, but doable.