As a marathon weekend attender, I can give my POV from last year (and my plans for this year lol). Overall crowds in the park were not terrible IMO. Busy, but not holiday busy. Your best bet to visit the parks in the morning, as most runners will be a) still running b) resting c) eating. Additionally, less runners will enter the park as the weekend continues (th is 5K, fri is 10K, sat is half and sun is full) because it will a) take longer to finish b) more of a toll, especially those running multiple races. Where you may actually see an increase in people is by the pools/hot tubs at the resorts post race, as that is where I plan to go immediately lol.
Edit to add: I also include MK as being “busier” Sunday and Monday as many people go to take pictures of their medals