I’ve done Epcot post marathon the past 3 years for this exact reason! Thinking about nachos and a margarita keep me going those last few miles
I haven’t found it to be too busy any of the years. No issues getting food/drinks/photos. This year I stayed to cheer until the end of the marathon, then went back to hotel to shower, and then back to Epcot, so I got there a little later. The previous 2 years, I went back to hotel immediately after finishing, showered, and got back to Epcot while the marathon was still going on. It’s my plan for next year too!