If there is nothing on the site, then email
disneysports@trackshack.com. I don't recall entering an estimated pace, but I've only signed up as part of a challenge. But that email is where they would accept times for placement. The corrals are seeded based off of estimated finish time, but there is like 5-10 minutes between the corrals, so it's fairly spaced out.
Running a local race is always a good thing. Odds are your local 10k won't have 9-10k runners. Even with the new corrals, each corral easily has as many runners as most local 10ks. At a minimum, the start area can get a bit full and take a few minutes to get up to stride. That said, I've both run a 44 min 10K on this course (solo), as well as a 90 min 10K on this course (with my DW). Epcot itself had some bottleneck where you enter and leave. (China and International Gateway) But nothing compared to some of the congestion I've seen on World Drive towards the later corrals from the half, or most of the DLR in park course.