We were just at the Boardwalk and the walk back to the Boardwalk from MGM was about 15-20 minutes. We were given misinformation about the path from someone who worked at one of the Boardwalk restaurants.
If you are at the Boardwalk, you can get to the MGM path by walking either to the left of the Luna Pool bar or to the left of the Conference Hall pool or behind Jellyrolls.
We ended up taking the boat from the Boardwalk to MGM. The boat first stops at the Boardwalk then the Beach/Yacht Club then at the Swan/Dolphin then to MGM. Once you were on the boat from the Boardwalk to MGM it was about a 20 minute boat ride. That does not include time waiting for the boat which seemed to come about every 15 minutes or so.