Just to give yourself a fighting chance of retaining your sanity, you should take open the MIDI file in a sequencing program and get rid of all the track except for the melody and the bass-- if there is a track with only block chords, you could use that--but beware, because, especially with modern stuff, there will be horn or reed parts with block chord phrases that complement or contrast the melody--

The other thing is that you have to trim the thing down to 12, 16, or 32 measures(depending on how long the melody is)--remember that the MIDI program will show you everything that is there in the MIDI file, which is way more that you need or want to have to deal with--