@Lyndon could probably answer this better as I’ve only read through the book a couple times, but it seems like 20 minutes is a bit excessive? Maybe they’re trying to teach things? Most of the pre-game seems to be seeing what’s on the table for possible scenarios and then checking to see if any rivals turn up to mess with your plans. If I remember right, scenery was pretty open to what you have. Opponents were suggested, but dependent on your miniatures selection. The real crunchiness came after the game. Rewards, injuries, training, etc.
the campaign event is that a zombie hoard goes through the area. Then roll on a chart before each battle to see if an event takes place, such as wild zombies or displaced survivors. But the battle events could have conditions that affect the campaign events, such as if you killed all the zombies before the end of the battle it represents people generally killing off zombies as they find them, then when the hoard arrives it has been partially depleted. 