Don't use the "Modify Results" part of the rule setup dialog, it will only cause problems. Just remove the length limit modifier you have - it's purpose would be to limit the universe of songs in your library from which tracks are chosen to add to the Play Doctor playlist. This modifier basically says: grab less than 5 minutes worth of songs from your library, then choose a random playlist from that selection, so it's creating the whole playlist from a universe of one song.
I think what you're trying to do is only include songs in the playlist that are shorter than 5 minutes, is that right?
To do that, click "Add New Rule" and set it to "Duration" "is less than" "300" . 300 refers to 300 seconds in 5 minutes.