We just left disney yesterday, so on the second to last day of the events leading up to the full marathon (which was today). We were there for 5 nights. Monday and Tuesday were not bad. This was our first trip in a decade so I'm no expert in crowds at disney, but it didn't seem too bad those first two days. We definitely noticed a pick up in crowds starting wednesday night and thursday, which I think is when some of the events started. Then Friday we definitely noticed things getting more crowded, though it certainly wasn't unbearable. Our hotel got fuller (WL), too. We left Saturday morning, and it was kind of a hassle because lots of roads were blocked off for the runners, and it disrupted some of the parks. You'd need to leave extra time to get around Saturday (and presumably Sunday too).
All in all, I didn't think it was too bad and I don't regret going last week, but it certainly wasn't dead in the parks.