The private pilot reported that this was his first solo flight in this airplane. During the takeoff roll, the pilot reported that the main cabin door "popped open" around 60 knots. Believing he had sufficient runway remaining to stop, he reduced power to idle and applied "maximum braking." Subsequently, the airplane overran the runway surface and came to rest in a creek about 72 feet past the end of the runway. The pilot reported that the braking action available was "insufficient" to stop within the remaining runway distance. During a postaccident examination, a Federal Aviation Administration Aviation Safety Inspector performed an inspection and functional check of the brakes and no anomalies were found. Additionally, the inspector found skid marks that were 501 feet in length before the end of the runway. The left and right wing both sustained substantial damage.