Agree, bus to MK. The boat landing is in front of MK entrance. Boat on over to the Fort and Pioneer Hall is straight on in front when you come off boat dock. I think 90 minutes is safe. If you get there early you can have a seat in a rocker on the front porch of Trails End restaurant.
Keep an eye on the weather if you plan to take a boat over to Fort Wilderness from the Magic Kingdom. On our recent trip the weather was bad due to TS Andrea. We drove our van to Fort Wilderness and had to take the internal bus into the campground to Pioneer Hall.
If there isn't a boat from MK I think you would have to take a bus to Fort Wilderness.