If you've body's spent most of it's life running in a different way it's going to take a long time to adjust
I switched to more of a mid-foot strike (maybe when I run fast enough I can have a forefoot one
). This change means the placement of your foot also changes. The position of the strike feels like it is directly below / slightly behind your backside. It took a couple of weeks for the change over. There was no added stress / aches / pains but then again I'm not training for a marathon. I would have thought it not a good time to change over or try if you are about to do one.

