I have never floated from M.C. to Sand Island, so I wouldn't know how long it takes...but I think it is more of a full day event, not 2-3 hours. I beleive if you asked nice enough you may be able to put in behind the really nice hotel/lodge there in Bluff, but I honestly don't know why you would want to do that...it is a lesser put-in and really wouldnt allow you to see much more of the river.
For my money, I would head across the street to Butler Wash Rd. and check-out one of the many side hikes. You have numerous rock-art panels, ruin houses and interesting things to see and do. Check out Hiking Guide to Cedar Mesa Book for the best hikes. A couple of these hikes would take a nice 2-3 hours, but would keep you close and in the area.
Just one opinion.