The shortest walk is out the internional gateway (if a boat is actually in the dock and you have children in tow or are a slow walker - take the boat) otherwise keep walking. Ignore the little path to the left along the water and take the wide path to the right. turn left over the bridge and along Boardwalk past ESPN, Spoodles and Flying Fish. Turn left after the Surrey Rentals, then right after Wyland Galleries. Go under the main building then sharp left along the path between the Boardwalk Villas and the pool area. This brings you to the main path along the canal to MGM. Turn left. You ought to be able to see Tower of Terror in the distance.
If you walk briskly, you should beat the boat as it has to stop at every hotel along the way
Andrew