Here is a tip for making posts set in the ground last longer. Drill a 5/16in. hole about 4" long and at an angle, just make sure that you don't go all of the way through the post. Fill this hole with Linseed oil and put a cork in it, refill every other year and it will help keep the post from rotting.
Saw this in Mother Earth News years ago.
That is going to be one fine coop and run when finished.