Q: After releasing gas pedal, car continues to accelerate

On a four-lane highway, I was in the right lane and moved into the left lane, accelerating under full throttle to pass a car. After passing it, I moved back to the right lane and when I released the gas pedal, the car kept going - it just kept accelerating! I pressed and depressed the full range of the pedal to make sure that it was free to operate, but that didn’t make any change. At wide open throttle, it was stuck. As hard as I could, I then put full pressure on the brake to try to slow the car down as it was continuing to accelerate. I could tell brake failure was imminent because they were getting hot. I shifted the transmission selector up to Neutral. After the engine momentarily overran to the rev limiter, it then settled to 4000 rpm. While it was in Neutral, I continued to press on the brakes to stop. It did stop and after 30 seconds the engine slowed to a normal level of idle. The throttle seemed to respond normally. After shifting to Drive, I headed to the dealership. Around 30 seconds after starting to the dealership, the “grade braking on” appeared and then went off. It was a flat road and I was going 20 mph and not pressing on the brake pedal. The tow/haul mode was not on. Before I agree to what the dealership finds, would you please diagnose all of this and give me your advice? Thanks.