We went during the marathon this year, and it really was not bad. The day of the half marathon, we did not notice anything, other then people walking around with their medals. We spent that day at MK. We arrived early in the morning, and they had the middle of main street roped off for the runners, but you could walk on either side, and it was kind of fun. They played music, and people where cheering them on. It was cleared away pretty early in the day, and made no difference in crowds really.
The marathon day we where in Epcot, where the marathon starts and ends, and it truly was exciting. Seeing the people at the end was awesome. We waited no more then 20 minutes for any ride, and that was without a fast pass.
I really would not let that be the reason you don't go then.