The pilot prepared the self-launching motor glider for takeoff by extending the airbrakes; this also engaged the wheel braking system. Using the taxiway as the runway, the pilot then proceeded down the taxiway for takeoff; however, the glider did not gain sufficient airspeed to depart. The pilot elected to continue the ground roll to an overrun area, in order to give room for other gliders to depart. The glider impacted a drainage ditch before coming to a stop. The impact resulted in substantial damage to the bulkhead and structural mounts for the landing gear. The pilot reported that he may not have had the airbrakes retracted and stowed for the takeoff, after releasing the brake.