We went down for the marathon last year. We arrived on Saturday and went right to Magic Kingdom. I think the longest line we waited for was Space Mountain and that's because it broke down.
The day of the marathon, we volunteered so I can't tell what it was like during, but after the marathon we went to MGM and it was fine. Practically no waits.
Then Monday the parks were dead. We went to all 4 parks and were able to get on everything we wanted.
So from my experience the crowds weren't that bad. Figure wise, I had the opportunity to speak with the marathon organizer last year. She said they were expecting about 24,000 runners. Add in family members and such, and divide that over 4 parks. Not too bad for crowds, I don't think.
Any other questions, I'll be happy to answer.