Marathon Weekend at WDW is January 5-7, so I'm surprised you were able to get anything at all for the weekend nights. Marathon weekend doesn't affect crowds in the park very much, but there's LOTS of runners staying in the resorts on Friday and Saturday nights. And on Saturday and Sunday mornings, when the half marathon and marathon are going on, you really *don't* want to be trying to get to Magic Kingdom or Epcot, because the roads are mostly turned over to the runners. Things should be back to normal by 11:00 or so.
Weather is typically in the upper 60s/low 70s, but it can be 80, or it can be 40, with freezing temps at night. You never know - come prepared for both.