To me, out of hand means out of control. "The economic crisis in Europe has got completely out of hand."
As far as your sentence goes, then I would say you have used the other definition correctly though.
I have added "about" to your original as you "talk about" something. An alternative would be "Ken and I have to talk all this through...." (to talk through = phrasal verb meaning "to discuss" and which doesn't require "about").
Student or Learner