DD and I spent the day in Disneyland Park after the Avengers' 1/2 marathon that Sunday. It was manageable~ later on in the day, it seemed more crowded, but lines actually weren't too bad and moved faster than wait times posted. Along with the runners, the Christmas decorations/overlays seemed to bring in the crowds, as I believe it was one of the first weekends they were up. We were able to get a lot done despite the crowds. I think if you stay late, the crowds may thin out being a Sunday night for the locals and the runners leaving early due to early running that day (at least we did). The Christmas decorations made it all worth it. Amazing!
I can say even with the holiday decorations, it still wasn't as crowded as Labor Day Weekend!
That was our experience, we have not been in August so I can't help compare there. Hope this helps a little.
Sunday morning was alright. Many of the marathoners just wanted to take pics with their medals in the parks, or rest for a bit. As the day progressed it got a lot busier, but manageable. I think that saturday evening was the worst as far as crowds.
I agree that the Sunday after the marathon was manageable, but the parks weren't empty (i.e., don't expect walk-on rides). The parade route filled up really early (but it did that on the weekdays after the marathon as well).