Do you need to keep the directory structer the same? If not, just do a search for *.mp3, select all then copy to external HD. Then you could load Itunes back up and add the external HDD to your itunes library.
that is my ghetto solution, but it's gonna be eeeeeeasy 🙂
I had a similar problem last year, so I spent a few weeks, lots of hours. tagging all my mp3's and making a directory structer. Now my mp3's are organized. Was worth the effort