Hello fellow Aussie
I'm travelling from Melbourne to run the half marathon this year. We went over that weekend last year too (ran the 10k), and I agree with most of the observations mentioned so far. Here are some things you might notice.
- As previously mentioned, the bulk of the crowds will start building from the Thursday. It will be busy by the Friday when you arrive. Majority of the crowds will head home on Sunday or Monday though, and the rest of that week is known to be one of the quieter times of year. My advice... spend the weekend getting your bearings and taking in the atmosphere. Make ADRs for any dining you want to do on those days and make good use of FP. Once the crowds disperse, you will find the parks very enjoyable for the rest of your trip

- If you want to get stuck into rides straight away, make use of early mornings and late evenings. Most runners and their families will be showering/eating after their run when the parks first open, and retiring early in preparation for very early morning starts. There is a private party on the Thursday night, but that won't affect you anyway. Apart from that, the park hours are not affected at all - they make sure they send runners off early enough (and catch any stragglers) so that it doesn't interfere with the day-to-day operations of the parks.
- The area around the Disneyland Hotel and Downtown Disney (especially that end of it) will be crowded until Saturday afternoon, as that's where all the running related things are - expo, start line, finish line, kids races etc. Also, a lot of people just come for the race and don't actually have park admission, so they will be enjoying the publicly accessible parts of the Disneyland Resort.
- You might not see any people actually running, but you will notice lots of people walking around in athletic clothes, and also plenty of people walking around proudly wearing their hard-earned medals once the races are finished.
- The last half marathoner may not finish until around 10am, but the parks are early on in the run. I would expect all the runners to be finished in the parks by 7ish. They will go around and pick up anyone who is too slow so that park opening is not affected.
- For the person wondering if they should focus on DCA - it really doesn't matter, the races don't affect one park more than the other.
Don't be concerned about it ruining your 20th anniversary celebrations. Make sure you stop by City Hall when you arrive to get anniversary buttons (and 1st visit ones too if applicable), and mention that it's your anniversary in all your reservations (hotel, dining, tours etc). You never know what sort of pixie dust might get sprinkled your way