To find the path on leaving MGM, bear right towards the buses, then swing left keeping the water on your left hand side. The path is lit all the way. Keep to the path all the way to the end for Swan, Dolphin, Beach & Yacht. You can save a couple of minutes by cutting through Luna Park pool if you are headed to Boardwalk lobby, Epcot, Flying Fish, Spoodles or ESPN.
Unless you run like fury when you leave Fantasmic! to ensure you are first on the boat, you may have to wait for a couple of boats before you get on. Unless you make that first boat, walking will certainly be faster.
As to time, I would go with the longer rather than shorter estimates. I use an electric powered wheelchair with a top speed of 5-7MPH. (about the same speed as a slow to medium jogger) Flat out, from the MGM turnstyle, around the end of the canal, past the buses, down the path and taking the short cut through Luna Park to the Boardwalk Villas entrance takes me ten to twelve minutes. You are going further, and unless you are running, you will be slower.
*And please, please, PLEASE, do not get up early to beat the crush out of Fantasmic!. They put us wheelchairs in the back row, and whenever anyone stands up in front of me (which inevitably happens) I cannot see the stage area. I have now seen Fantasmic! theree times, but I have never seen the ending.
Andrew