The pilot reported the engine performed normally during the engine run-up; although he needed to apply a little more power than normal while taxiing. He stated that the airplane did not gain sufficient groundspeed during the takeoff roll to become airborne and he waited too long to abort the takeoff. The airplane traveled off the end of the runway and down an embankment resulting in substantial damage to the aft fuselage. The pilot stated he did not notice a loss of engine rpm during the takeoff and that he had little experience operating on grass airstrips. The airplane sat outside for two days prior to the accident with the parking brake engaged. The position of the parking brake at the time of the accident and the tire pressure could not be determined during a postaccident examination of the airplane. The examination did not reveal any mechanical failure/malfunction.