Have a look at a map of Fort Wilderness. The boat dock is on one end (the Settlement area) and the trail rides are at the other end (Outpost area).
The boat ride itself from MK takes 15 mins. Factor in an additional 20 mins for waiting for the boat/unloading/loading. Once you get to Fort Wilderness, you will be in the Settlement area. Follow the walkway to the depot for the internal buses (a few mins walk). There are 3 colour-coded bus routes. Good news is any of the internal buses will take you to the Outpost and, therefore, chances are good you won't wait long for a bus. Factor 15 mins to get from the Settlement to the Outpost. I would say from the Poly, minimum 60 minutes, and that would assume no waiting at any transportation segment.