In the past, runners don't go toward the MK in half marathons for Star Wars weekend. AK, HS, and Epcot are the areas. The route has been different each of the 2 years and 2018 has not been announced yet. We hit HS just before opening, then Epcot after it opened. Folks could enter each park, but would be delayed a few minutes as the runners pass.
In terms of traffic, based on last year's route, the easier park to get to with less running traffic would be the Magic Kingdom.