I did a glitchless TAS of SMB, and I got the final number of frames, divided it by the framerate of the NES, and got a time way faster than will ever be acheived in the game. What am I doing wrong?

Well it's tough to tell without knowing the frame count you got

I got 15901 frames, divided that by 60.098813897441 (NES's framerate) and got 4:24.580 seconds. Impossible time by a long shot.

Hold on, I just realized I was using a PAL rom with NTSC timing. Not sure if that has anything to do with it, but retiming it with PAL framerate yields a 5:17, a much more reasonable time.