That looks more like a pecan, or hickory a lot more than a california walnut. Would you be able to take a photo a little bit more zoomed in on the nut, it isnt clear enough for me to 100% id. It could also be a the seed of a feral pecan, there is an old tree of feral pecan on my farm. If you like, I have some really good improved varieties of pecans that I can give you some budwood of this winter if you want to graft it and get some really good nuts, but they are huge trees.