It is converting whatever editing format that you used in iMovie to the mpeg2 dvd format. This is going to take a while depending on what computer you have.
I just completed a 2-hour 26-minute film project. I edit at 1920x1080 in Pro Res format in FCP. It took 12 hours for Toast 11 to render it out for Blu-Ray HD-DVD, and about 2 hours for it to render it out for a standard def DVD.
I am running a MacPro tower, dual-2GHz dual core with 9GB RAM.