It is fair to point out there is a very, very small percentage of material that will actually give you distinct sound to 7 channels and of those titles on blu ray I've listened to the difference has been so subtle I wouldn’t' go to great lengths to do 7.1 for myself and I have a very similar room size to you. In larger rooms as already mentioned it can serve a purpose to help fill the sound out in the room, but for the point of hearing 7 distinct channels I don't think the source material has a high enough penetration right now to bother.
As far as fishing the wire, is your ceiling finished? Dry walled? Drop ceiling? Do you have access to it from an attic maybe? If you can get access it isn't to bad to fish down insulated walls if there is some sort of vapour barrier or something between the wall and insulation... if there isn't then it can get a little tricky... here's hoping you don't have any horizontal bracing on your walls or you're in big trouble
I actually pulled my baseboards off and fished from my baseboards up for my surrounds and ran my wires behind my baseboard. Or rather I fished from the hole I made, down to the baseboards.
