First, I would fill the tank yourself (it may not have been completely filled by the dealer) and reset the trip odometer. Then, drive a few hundred miles in mixed driving conditions (city, highway, etc.) then refill the tank and check the mileage. Do the math yourself and check it against the car's trip computer; computers are always accurate.
Yes, it may take up to 1,000 miles to break in the engine though break in isn't as necessary as it was 40-50 years ago; modern engine tolerances are much better. Mileage should improve with time. As they say, YMMV.
Second, which trim do you have. L, LE or XLE? Check the manual for adjusting the seat height.