Converting the file to mp3 will make the file smaller, but it won't make it any shorter. It will still be 90 minutes, meaning the length is still too long. Besides, it will have no effect on burning a cd, because audio cds only use CD-A files (similar to WAV). A burning program will just take your mp3 file and convert it right back.
Converting to mp3 will only work if you burn it as a data cd (or mp3 cd). However, the downside to that is that you can only play it on a computer or a cd player that plays mp3 cds (and the majority don't). If you want to play it in normal cd players, you're going to have to split it into 2 cds. Download yourself an audio editing program, and split the track, or cut a part off and paste it onto a new file.
Don't Jimmy me Jules,
Bri