You can split your large mp3 into small pieces by using the "split" or "split new" function in audacity, then can move apart the portion of audio that you wish to mix down into a different file in order to export it.
In the next screenshot, I have made a new Stereo Track in order to put the split audio into a different track (for visual purposes only), then I moved the second part of the audio to that track and I have selected a portion of audio which can be exported into a new file.
You don't have to split the audio in order to export the portion of audio into a different file, you can simply select the audio and choose "Export Selection" in the File Menu.
In the next screenshot I am doing as I said in the words above.
If you yet have the CD, ripping audio with a file per song can be done by using "Sound Juicer", which will allow you to choose how to rip the media from the CD. This way you will have an mp3 file per song. And the task to mix down all the songs into a large file will be easily achieved by Audacity, just in case you need to do it so.
Sound Juicer can be installed via synaptic or by diving into the website at: http://burtonini.com/blog/computers/sound-juicer
A screenshot are placed here in order to you see Sound Juicer in action. (Sorry, I don't have a CD to rip off right now, so the playlist is empty)