This may sound counterintuitive, but at all the schools from which I received Dec. acceptances, I interviewed less than optimally. That is, I stuttered quite a bit and more than likely came across as anxious and awkward. If your stats are above average and you were genuine with your answers, then you are probably fine.
Multiple um's won't break your application culminated from years of (presumably) hard work and dedication. Sit back. Relax. And try to enjoy the process.