Hi, Paige! Even if you're in half as good of shape as I am (go ahead and laugh,) it's a short walk from the International Gateway of Epcot to the BoardWalk. I'd estimate a good 15 minutes of hoofing it (give or take.) You could also opt to take the boat from the Gateway to the BoardWalk if you'd like to give your feet a rest. There's no shame in that. In fact, I've been with
Disney Dad Marc as he took a powernap on said boat.
Your "Plan A" return trip might be more difficult. Not because you've enjoyed your adult night a little
too much - rather, its success hinges on the park hours of Epcot. If it's after park close, you'll have trouble getting back into Epcot. That doesn't mean you're without options though.
You could take a taxi from the BoardWalk back to the Grand Floridian. Or, you could take the boat from the BoardWalk over to Hollywood Studios and then grab the bus to the Grand Floridian from there. I believe either option would constitute as "working off dinner," right?