If you have an ADR, you'll be told to be at the podium to check in 5-10 mins prior to your ADR time. I would actually leave you room, at the Polynesian, 30 mins before your ADR. That will give you time to wait for the monorail, ride to the CR, and then checkin.
If you have plenty of time, take the boat to the MK. Once it gets to the Polynesian, it goes right to the MK. Then, if it were me, I would just walk to the CR.....under 10 mins. I would leave 45 mins to get there using this approach.