With only a few exceptions, no resort to resort trip can be made in one go. Boardwalk to AKL has no direct link. Provided the parks are open, your best bets are to change buses at Animal Kingdom or bus to boat at MGM. MGM to Boardwalk is a 15 minute walk if you don't want to wait for your connection.
The trip between AKL to Boardwalk takes a while, but I would hardly describe it as a 'horror', at least during the day. A taxi would be marginally faster, but wouldn't be free.
Once the parks are closed though, you will have to change at Downtown Disney which means passing within sight of the Boardwalk on the way to DD and then having to come all the way back again. Now that *IS* a long ride.
Andrew