I assume you mean recorded a 90 minute set as one long song using a single track?
If you want to break the set up into separate songs on the DP 24 without using
a computer, just use the menu command to make a copy of the song,
edit the copy so it has only the 1rst song in it. Do this by setting the in mark at the beginning of the first song, and setting the out mark at the end of the recording. Then use track edit 'cut' everything from the in mark to the out mark. This will leave the copy with only only the first song in it.
Copy the original song again and then using the in and out marks, isolate the second song, and so on.
So if you have 4 or 5 songs in the one lone song, you will have to repeat this process 4 or 5 times.
There are other ways,,, to do it without a computer, but most ppl find this one easy (but a little tedious).