You won't see much of any character lines from Corral A. The majority of the faster runners don't stop for character pics, and even when they do it takes like 5-10 sec at the most. Almost never lines. This youtube video is from RunArnoRun's 2020 HM in which he started at the very front of Corral A, maintained just about 2 hour HM pace, and he typically stops for most every character.
This Arno video is from the 2020 M and he ran the first half at about 10:30 pace.
Lastly, this Arno video is from 2018 M and he ran it just under 4 hours. First 5 miles in under 7:30 m/m, 7:45 through 10, 8:00 through HM, 8:52 through 20.
Based on past races, about 5-7% of runners at the runDisney marathon will finish under 4 hours.
ETA - I will add that if someone goes faster, like 6-7 min/mile pace, then I've had many occurrences where the characters aren't even out yet and/or the photographers aren't prepared for people. So it can be hit or miss sometimes in case someone else in that pace range is reading my past experiences.