Surprised? Maybe, sort of, not really.. Just learning by doing, and TIL that a linear voltage regulator will not work for the application I wanted. Measure once cut twice! 
I'll just drop the 12v adapter and take the 5v PSU from my RBPi for testing and order an extra 5v adapter for the final assembly. Just had the 12v adapter and a 5v regulator and though: That might work.. Well.. It didn't! 