In general terms, our heroes and villains are out in the sticks here. The plot benefits from travel times being long, quite regardless of whether this is because distances are long, or because travel across certain stretches is slow for some reason.
And 12 hours is in the Goldilocks zone, sort of. Any longer, and Kirk might just as well turn back and ask for somebody else to deal with Khan, as the time would be too big a percentage of the total of two-three days from point-of-summons to Regula, and "they are the only ship in the quadrant close enough to do the job" would no longer apply.
Any shorter, and the sense of isolation would be diminished. Plus, Kirk is justified in thinking this may be a mere clerical error and technical fumble if he spends a day or two approaching and then does a final set of hails half a day before reaching the destination - but if he gets to within an hour or two and still hasn't made contact, he really ought to be declaring galactic emergency and preheating his phasers.
Timo Saloniemi