Another Steam/install problem..7 hours download, really?

[gkhorse]

Sorry folks, I searched but didn't find an exact address to my question but I apologize anyway because I KNOW I can't possibly be the first with this particular question/problem. Help appreciated.

I have the DVD disc of COD. When I attempt a first time install from the dvd drive all goes well quickly enough and then I get the Steam gray box that gives me the check box 'option' of 'play game when ready'. Then it begins a 'download' that tells me will last approximately 7 hours and 15 minutes...thereabouts. I don't understand. Opening the disc appears the sim is on the disc. So, what exactly am I downloading for 7 hours and 15 minutes? What am I doing wrong or what do I need to do?

Thanks for taking the time to help.