>Frankly, anybody who has a D600 should get to the Manual, Shutter Priority or Aperture Priority stage, from the very beginning.
I must respectfully disagree. There is nothing wrong with using P or as it is called "Flexible program." All you have done by selecting P is to accept the meter settings and the suggested combination thereof. From there, you are free to rotate the command dial to pick your preferred shutter speed or F stop. It is not going to change your ISO, even with auto ISO. During a fast moving shoot, I wouldn't hesitate to use P, but if you decide on M, Manual, you had better be watching your meter, especially if you don't have Auto ISO turned on.
I also must respectfully disagree with your '"training wheels" analogy. Although I seldom use P, there have been many times that I was happy to have my flotation device handy so that I didn't drown.
So, my message to the OP is, "don't let your ego get in the way of your photographic aspirations."