The pilot inadvertently allowed the aircraft to make its initial touchdown on the two inch-high lip of the displaced threshold. As the left main gear tire impacted this lip, it began to rapidly lose air and go flat. As the landing roll continued, the aircraft started to veer to the left, and the pilot was unable to keep it from departing the side of the runway. After it departed the runway, the aircraft's left main gear encountered soft terrain and experienced an overload collapse.