It seems to me the bus is new, because last time we went, the drop off spot (at HS) was away from all the other bus stops.
As others have said, usually he fastest way to get from HS to the back entrance of Epcot is walking.
With the boat, timing is kind of everything. It may make sense to take the boat part of the way. If you walk, you can cut through the Boardwalk pool area.
That said it is a healthy distance to cover on foot. Some say 15minutes, but I timed it last visit, and even at a pretty brisk pace, in ideal weather, it took us a solid 20 min. In summer heat, the trek may well wipe you out, especially if you are continuing on to TSM or Test Track.
However, as others have also said, the back exit of Epcot is just that. The boat launch is at the back entrance of Epcot. If you are at the very front of Epcot, it may be faster to take the bus. Of all the parks, Epcot is the most spread out.