MP3 files are generally sampled at 128kbps and as such are the best size to sound quality for efficient file size in a computer.
However if you convert back to "audio" format your songs will be recorded onto a CD as analogue signals and will be as long as the song is long - a 4 minute song will occupy 4 of the CDs 70 available minutes.
This means on average you will only have about 20 songs on one CD.
If they were put on the CD as MP3s you could fit 200 songs on average.
I point this out because you wont be any better off simply because the songs are MP3s - the analogue recording will see to that.
Using Windows Media Player or WinAmp OR ANY OTHER DISC-BURNING SOFTWARE select the tracks you want and burn them AS AUDIO TRACKS (the files will have an extension of .cda).
Remember: you can only fit about 20 tracks onto your CD.