The resort monorail has to make 3 stops to get to the Polynesian (MK -> Contemporary -> TTC -> Poly). There's also a resort boat doing a loop (MK -> Grand Floridian -> Poly -> MK). If the boat is already at (or near) the MK, it is probably the faster trip. If the boat is not already there, I would head up to the monorail since it comes more often than the boat. With either option, you should have no trouble getting the next ride because there shouldn't be much of a line. From the entrance of the Magic Kingdom, I think either option will be under a half hour travel but how long it takes to get out of the park will depend on the crowd level.