The pilot stated the approach was on the glide path and approach speed was approximately 85 knots. Several minutes prior, the manifold pressure gauge became erratic and was unable to be used as an accurate indication. He pushed the manifold full forward for the remained of the approach. The landing touchdown was 1/3 down the runway and seemed to be a "tad fast". Halfway down the runway, he realized the engine wasn't going completely to idle. At about 2/3 down the runway, knowing that the airplane was too fast, he looked at the option of going around and ruled it out because of the slow speed and the height needed to clear the tree lines. He attempted to slow the airplane by applying full breaks; however, was unable to slow down before running off the runway. The responding FAA inspector observed the accident airplane lying off a berm, in a nose down position, approximately 50 yards from the end of the runway. The damage consisted of a bent propeller, buckled nose gear, damage to main gear doors, wrinkling of the right wing between station 147.75 and 163 and damage to the wing spar, and a dent in the leading edge of the right horizontal stabilizer. The pilot stated to the NTSB investigator that there were no mechanical issues with the airplane and he should have initiated a go around. The published length of runway 18/36 at the Freeway Airport, Mitchellville, Maryland, is 2,420 feet by 40 feet. A completed Pilot/Operator Aircraft Accident/ Incident Report was not received for this accident.