I'd imagine her tank was around a quarter of the way empty. If they couldn't find the keys and drive the car, they could siphon gasoline out of her car. They should have at least tried before getting trapped in the house.
shareYeah especially early on before the surroundings became infested with too many zombies, Ben and Barbara could have easily made it back to the car.
Maybe there were too many zombies there. It was after all a cemetery.
They were moving on the fly so I'm only judging their actions or game plan from the time they thought of it forward. There must have been other neighbors with cars or a few vehicles at the cemetery. So the reason would have been the danger of venturing out that far and then getting back. It is before Barbara realizes how slow they are too.