Foot pronation is a running speed killer. When the foot flares out during your running gait you will be putting on the brakes with each foot strike of the pronated foot. You will need to stretch your piriformis muscle on the affected side and use a good corrective orthotic.
If you think you have foot pronation see diagram above go to Barefoot Science to learn how to correct it.
As a chiropractor working with athletes helping them to improve their performance, I used a couple of different types of orthotic inserts. From custom made orthotics to off the shelve brands. What I have learned over the years those orthotics I used previously did a good job in supporting the arch but that was it.
The Barefoot Science orthotic will stimulate the proprioceptors in the foot which turn the muscles of the arch back on to recreate the arch. They also will enhance the athlete's balance by the proprioception stimulation communicating with the brain.