we've been a couple of times in January during the marathon, although not this past January. It never seems to affect the crowd levels in the parks that much. The race does wind through the parks and it can briefly affect your ability to get around within a park, since the CM's will hold pedestrians back briefly to let the runners through unimpeded. But it doesn't take long for the bulk of them to pass through. And it can jam up vehicle traffic if you're driving among the parks and resorts or taking a bus, but again it clears up quickly, usually by noon. Weather in January is variable, and it can be quite chilly, but I guess we've just been lucky and have always encountered moderate to warm weather. The best thing to do is check the long range forecast a few days before your trip to see what the temperature trend is going to be.