Hi there, Kristin!
Disney's
complimentary transportation will stop running one hour after Park closing time. In theory, depending upon what time Magic Kingdom closes - the Park doesn't always close immediately following fireworks, for example - you might be able to hop aboard a monorail to Magic Kingdom and, from there, take a bus back to Disney's BoardWalk Inn.
Because Epcot typically does close before Magic Kingdom, it is not likely that you will be able to take advantage of the monorail to Epcot and a walk through to the International Gateway in World Showcase in your Resort.
In all honesty, you might be best served with a cab. Taxis are readily available throughout Walt Disney World, thanks to Disney's relationship with Mears. Just ask the concierge or Bell Services to call you a cab when you are ready to return.
There's even a handy fare estimator, which will give you an idea of the relatively low cost of this option: http://www.mearstransportation.com/service/taxi-fare-estimator.asp
Have a wonderful time!
See you later, celebrator,
-Deborah B.