The official pace requirement is 16 minutes per mile, but that starts with the very last set of folks cross the start line (the infamous "balloon ladies"). In general, you would have whatever additional buffer on top of that based on how far ahead of them you start. But, usually, there is no sweep during the 5K! My mother (who has knee and hip issues) walked the Princess 5K a few years back in about two hours. There were plenty of folks behind her and we were never told to pick up the pace by anyone.
ETA: If you're truly at risk of the course being closed, Disney will close off the character lines and waive people past, or tell those in line that if they wait they risk being pulled. But again, with the 5K you usually don't have to worry too much about this as there is no hard sweep. There's just a time by which they must clean up the course to open the park.