As others have said, you have 3 hours 30 minutes to finish after the last person crosses the starting line. So if the last person crosses the starting line 10 minutes after you do, you really have 3 hours 40 minutes to finish.
Disney does have corrals based on projected finishing time so that faster runners start at the front and slower runners start at the back. I know for the half last year (or maybe it was the full) you had to submit a "proof of time" from a past race in order to not start in the way back. They are reasonably good at enforcing the corrals on race day, so you can't really plan on finishing in 4 hours, yet sneaking up to the front to gain extra time. You will get a bit of extra time, though, so the 3 hours 30 minutes isn't a strict cutoff.