Walnut is an excellent exterior wood due to its rot resistance. This, like all rot resistant woods, is restricted to the heartwood. Sapwood won"t last long.
I"ve cut standing dead walnut in my woodlot that died so long ago I didn"t know what it was until I saw the color of the saw chips. Still sound many years later.
I never bother to have firewood cut more than a year in advance, give away all the rest. Apparently like you, no shortage here and we don"t need much.