To answer your questions:
you screwed up...That's for sure, everybody does form time to time, I guess...learn from it and I ask you, did you lean for the runup? No, I did the runup with mixture rich
why didn't you do your engine run up with mixture rich?
did you not do a run up/magneto check? Yes, I do it prior every departure
was there a delay between the runup and takeoff? Yes, due to heavy traffic. I leaned after the run up and then forgot. Not good!