The airline transport pilot was taking off from an off airport site on a personal local flight under Title 14, CFR Part 91. The pilot reported that he used a range finder and walked the takeoff area to determine that its length was about 1,000 feet. He said the airplane reached 45 knots airspeed during the takeoff roll, and he increased the flaps to 30 degrees. The pilot said the airplane did not accelerate further, and he applied back pressure on the control to take weight off the main landing gear. He said the airplane did not become airborne prior to reaching the end of the runway, and the main wheels struck a trench, collapsing the main landing gear. The pilot reported the airplane sustained substantial damage to the fuselage during the accident. He indicated that there were no mechanical problems with the airplane that contributed to the accident.