That distance is going to be tough. Is there a building a shorter distance from the far end of the paddock where a relay router could be put? Is there a building with power at the far side of the paddock? Is that power on the same circuit as that on the building with the AirPort Extreme? If so, then you may be able to use Powerline networking. It might also help if you can put the AirPort Extreme on a stand so that it can "see" out a window toward the paddock.