We've gone a few times during the marathon. We stayed in the Boardwalk area where the marathon comes to an end. In the parks and in our case, in the resort area, you will see the ropes and course and then, runners. It is a tad inconvenient but if you happen to witness part of the run, it's inspiring and emotional. To see all of these people pushing themselves to such limits for reasons as varied as the runners. You see the young, old, those in tip top shape, those not so much, some run alone, some run in groups, wearing mouse ears or tutu's....my eyes welled with tears more than once. The days following the race the parks seem busy and you will see runners visiting the parks wearing their Marathon Medallions everywhere. They are humble and gracious when you tell them "good job!" By midweek, you see far less evidence of the runners or the marathon. Crowds seem to return to "normal" for January.
Besides the marathon, January considerations should include the weather that can range from 50 - 80 degrees in the same week. Being from the Northeast, we've never had reason to wear winter coats around Disney but you'll see plenty of people doing so. The other noteworthy element in January is what I fondly refer to as "The Brazilians...there are a brazillion of them"! Large groups of young people travel to WDW at this time of year because it is their vacation time in their country. They are young, pretty little things, boisterous and full of energy, and they visit WDW in droves. Around the parks, you'll see flags herding the groups from point a to point b. Because we are early morning park goers, we tend to avoid the large groups. If we do come upon them in the park and they are going right, we go left.
Boy have I rambled! Sorry for the overabundance of info. Happy planning!