We stayed in the Beach Club Villas in August and it took us just over 5 minutes to get from the enterance to the beach club to the boardwalk. If you are in the Yacht club it takes a little less. We managed to walk from the enterance to MGM to the beach club in around 15 minutes. I'm not sure of the actual distance from one side of the lake to the other but it is not far.
It took less than 10 minutes from the Boardwalk to BC/YC pushing a baby stroller. Walking past the ESPN bar (or whatever it is) was really loud and annoying, we went past the Swan/Dolphin after that. Kind of a nice way to see all the different EPCOT resorts from different angles.