Inside Star Trek
repeatedly lists the 78 minute running time (for an intended 90 minute time slot), although it isn't that long on home video. I wonder if that is an inaccurate report of the running time or the result of editing on the current version?
Even if the book gets it wrong, 11 minutes isn't insignificant on a television shooting schedule. Keep in mind that the first pilot took roughly 16 days to shoot; the second pilot took roughly 8 (although Solow and Justman say 9, IIRC).