I haven't been at WDW during the triathlon but have been there a couple times during the marathon, which I suspect might draw even more people. It didn't seem to affect crowd levels much at all. The marathon is in January, during low crowd times anyway. There were some logistical issues regarding the runners going through the parks, and it did snarl traffic among the parks for a while that morning, but that was temporary. I would think the Triathlon might be similar.