It's about a 10 minute walk. I did it the last time I stayed there.
I'm not the most active person so I found it more tiring than I want before I spend the day running around the park. 
I found it more convienant to walk to the Dolphin to catch the boat there.  Shorter boat trip, shorter walk. 

 I would also get off there.  I would definitely beat the boat since it would stop at the Yacht/ Beach Club before it would get to BW.
The Boat from BW to MGM goes like this: EPCOT -> Boardwalk -> Yacht Club -> Swan/Dolphin -> MGM
The Boat from MGM to the BW goes like this: MGM -> Swan/Dolphin -> Yacht/Beach Club -> BoardWalk -> EPCOT
The path follows a canal and goes under a bridge which is the highway, so no danger of being run over. 
The walk to Epcot is shorter, I am told, but the boat from BW to Epcot is direct with no stops, so I didn't bother to walk to EPCOT.
Hope this helps!!
Cindy in NY