It's really a shame that you can't walk from WL to the TTC. It would be nice to just walk over and catch the Epcot monorail from there. But between the wildlife and no sidewalks, there's no way to do it now (at least, not if you value your life).
Really, the fastest would be a taxi from WL to BC, and then walk into International Gateway. But that is an expensive option when you have complimentary Disney transport available. But if you have a dining reservation, and your time is short, that would be the quickest method.
After that, the bus is the most direct and fastest form of Disney transport.
If you take any other combination of transportation, you're probably looking at about an hour. There are several combinations you can use: boat to either MK or CR, monorail to TTC from CR or MK, bus from MK to BC and walk to Epcot, etc. But you have to allow a maximum of 20 minutes wait time each time you have to make a connection. That's why the bus is faster.