Depending on where your room is and your pace, it can be a 10-15 minute walk. It has never been crowded when we've been there and always seems to be a quicker alternative to the multi-stop boat.
Last summer, we stayed at the Swan and one night, after Fantasmic, we walked back to the hotel. It only took about 10-15 minutes. Its a very nice walk, and I think its quicker than taking the boat.
You walk all the way to the end of the BW - just past Atlantic Dance - and go left. I think they do have a directional sign as well. If your room is on that side of the BW you may be able to just go out a side door.
Twice when we did this walk in January, we came down to the BW at the same time that the boat was stopped and picking up passengers. Both times we arrived at MGM just ahead of the boat - and we are average paced walkers - so I think the times are comparable unless the boat doesn't happen to make the stops at the other resorts. Either way it's a nice pleasant walk and a godsend when exiting after Fantasmic at night!
I walked from Yacht Club to MGM several times. It can be done from there in a brisk 15 minute pace. It would probably be a couple minutes shorter from BW. It is a very pleasant walk and beats the heck out of the boat which seems to go in reverse. You could probably swim to MGM faster than the boat ride. Try it and see. Have a great time.
Yes the path is lit at night and is actually very lovely. They have lamposts that have those 100 years of magic banners attached that sparkle for added effect.
CamColt,
Sounds like your plan is a good one to me - have a great time!