Personally, I wouldn't go to MK beforehand if I had a reservation at the time you mentioned. By the time you get through the turnstiles and head up main street you might have the chance to do one or two attractions before you have to turn around and head back out. It usually only takes 2-3 minutes of travel time on the monorail from MK to the Contemporary. However, it depends how long you have to wait for one to show up. They are usually running frequently at that time of day. I don't know if you are aware, but you can walk from the Magic Kingdom to the Contemporary. That is about a 10 minute walk. If it was me, I would walk and avoid having to line up and wait on a monorail.