Hi,
I read through your debrief. Although 680 is a good score, I know that you can do much better than that. I don't think you over estimated your abilities. I think you under estimated the GMAT
In my opinion, your score was pulled down due to inefficient time management. You had to guess the last 6 questions in the quant section, right? 6 questions is nearly 16% of the total questions! Had you managed your time more effectively, your quant score would have been atleast 2 points higher.
For verbal, Mohater says "Your total score is only as good as your weakest area." I totally agree with him. So, you can start by recognizing your weakest area and then begin working on it. Take prep tests throughout the preparation phase to make sure that you're making the desired progress.
Re-take is a very good idea and I'm sure you will hit +700 this time

Good Luck!