According to the pilot, he was overflying the airport when he had a sudden urge to go to the bathroom. He circled north of the airport and performed a straight-in, stabilized approach to the runway with 25 degrees of flaps. The pilot reduced the power as he crossed the runway threshold and made a normal landing. According to a witness, the airplane landed approximately halfway down the runway and was traveling very fast. The witness stated that the brakes were applied, the airplane swerved, and then it departed the right side of the runway. The airplane traveled down an embankment and came to rest in a ditch resulting in substantial damage to the wing spar and fuselage. The pilot reported there were no preexisting mechanical anomalies with the airplane.