Problems with the New iPhoto update

I just updated iPhoto and now I get the following message, "Your photo library will not be readable by previous versions of iPhoto after the upgrade. The upgrade process for very large libraries may take an hour or more to complete." I'm afraid to continue because, stupidly, I have not backed up my photos (I will now!). I've been reading similar stories after an iPhoto update last year, and the process has taken way longer for some - days and weeks and months, with no fix. I realize my negligence, combined with this update, has put my photos at risk. Does anyone know what I can do? I'm a beginner at all things Mac. Thanks for your help!