Congratulations on your trip and what a wonderful reason to celebrate! I hope you have a wonderful time!!
I have been for this weekend before. The Princess Half this year is on the 25th. It's not bad though. The crowd numbers you are seeing are because the 25th will be the last Saturday for the hoards of folks who went to WDW for Presidents Week vacation. They will be clearing out mainly that Sunday (the 26th) and the numbers will drop much lower.
The marathon itself doesn't make the parks crazy. The only thing it does affect is the travel routes to and from the parks and that can cause delays. But since you will be at a monorail resort it should be fine.
I have a story.. the day of the Princess Half in 2011 we went to Magic Kingdom for rope drop. It was EMPTY that morning. In fact, my husband, son and I rode Haunted Mansion.. BY OURSELVES. It wasn't even the first ride we went to. We did Splash and Thunder Mountain first and then headed to Haunted Mansion. We were the only people in line, the stretching room, and on the entire ride!!! Very surreal. It picked up later in the day.
Now this year will be busier because of the holiday week ending, but once those crowds are gone it will be smooth sailing.