This one is a new race. There is no after race party for this one (only Wine and Dine has a party now).
It is only a 5K, 10K, and Half and all happening overnight/early morning Friday, Saturday, and Sunday. So this will be about half the crowd of marathon weekend.
The biggest thing to factor in is traffic delays. The morning races are harder to tell how they will impact regular guests because most runners will be asleep early and out of their rooms around 3-4 am for a 5:30 am race start. The half is the only race that may affect parks but we finish in ESPN so I imagine everyone will be forced to be out of the parks by park opening.
With that said it will be hard to tell if runners who aren't doing another race will be in the parks late or early. It seems to be a split on how runners spend their day before and after a run. A lot of runners come just for the runDisney event and do not actually go to the parks. They spend time hanging out at the resorts and boardwalk area.