We ran the princess last year and here's what I will say:
-If you can, try to go pre-race weekend, most people tend to arrive the day before/of the expo, and do more park time post race weekend, as you don't want to tire yourself out!
-MK WILL BE PACKED on the day of the 1/2 marathon, we opted to do Animal Kingdom instead and it worked out well!
-Utilize fast passes, crowd calendars to which park each day, and realize you won't accomplish everything!
-Rope drop is important, esp on the days of the races (esp the bigger ones) the earlier you can get into a park, the better, because more people will still be running (i.e., use am EMH, then if it gets too crazy, park hop elsewhere)
-PARK HOP! Use the app to check wait times at other parks if you feel yours is too crazy, if one seems less busy, maybe check it out!
-Epcot is a great park for not feeling the crowds since its so spread out, and there's lots of "wandering" to do!
-Take some breaks when you run out of things to do/when lines get too long. We like to arrive at rope drop, hit as much as we can, eat lunch, watch the parade, and then peace out to the monorail resorts, enjoy some wine at the Gflo while listening to the band, some nice cocktails at Trader Sams, and maybe even take in the view from the Top of the World lounge at the Contemp, then head back into MK when the evening festivities start! We find that there's not much going on between the end of Fof & the beginning of HEA, and the lines for rides are usually at their worst.
-Avoid Disney Springs on weekends!