I was wondering about that too. We still don't have much info about how distance on the strategic map affects the distance on the tactical map. It doesn't really need to be a number of turns delay for units to arrive. They can be placed farther away on the tactical map.
I expect tactical maps won't be too different in size to Planetfall. If they were much bigger there would be a lot of turns before anything happened in every fight.
The other alternative would be to have them lose 1AP on turn 1 for every hex away from adjacent they were, so they've rushed to arrive in battle and now they have to catch their breath.