Not sure which version of the Nero you have Mr. D. I can tell you that there are two ways to backup MP3s onto CDR.
1.As a music CD - readable in any CD player. The music CD method has a 90min playing time limit (or thereabouts) and does take a very long time to burn.
2.A data CD would only play in CD players designed to read MP3s saved as a PC file or in a computer. It is generally written very quickly. You could literally fit hundreds of the files onto 1 blank CDR. (700mb)
Try opening Nero and choosing Data CD as type to create, then as FB suggests, drag and drop the lot of them onto the Nero window.
Good Luck !