This seems trickier than it looks... depends on how big the original recording is vs. your sd-card size, and if you can access the card with a computer.
First thought was to simply leave it as one large 'song', and just set the In & Out points for each sub-song before mixdown & mastering. But the machine only maintains one master file per song so it would only work if you can copy out the master file from the Music folder (or rename it) after each sub-song is processed.
If you don't have a computer, you could use the Song menu to copy the original song to a new song, set the In & Out points and master this. Then copy the original song to another new song and repeat. Each song will have its own master file.
If you run out of room, you could delete the unused sections of each new song before processing.
Having access to a computer would speed up the song copying, and you could also archive the original song first for safety.