When you walk, you have one leg in front of the other. Just like that, swing and exaggerate the arms one at a time.
The body is meant to “pump”, alternate and reach.
We don’t do enough forward and overhead reaching. We have gotten our body onto poor patterns so we extend through our lumbar spine/back.
Over exaggerate with forward and back arm swings and overhead reaching. However, be sure to keep your rib cage dome shaped and compact while doing these movements – without overextending the back and jutting out your ribs.
Enjoy your exercise walking!