I myself have never run a race with the intent to board a cruise the same day, but I would say factor into the equation if you are a slower runner or if you do not end up with proof of time, etc. that like someone else said you will be in one of the last two corrals. That means 20,000 people ahead of you all trying to do you what you're trying to do: finish, change, leave the race area etc.
Assuming that it takes an hour for the very last person in the very last corral to start, that is already 630 in the morning. I just picked that because it's an even one hour after the first corral should start. I do not have an idea what pacing would be for someone who is in the last corral but, to make things simple let's assume 3 1/2 hours. That is 10 AM that person would be crossing the finish line. Assuming you don't need the medical tent, or anything else, it would probably be about 1030 when you got to your car or got on the bus or the monorail etc.
Me, if I was trying to put myself in the best situation possible, I would make sure that I had a proof of time to at least put me in the middle of the pack. number two, I would have a car in the parking lot so that I did not have to depend on Disney transportation to leave the race area. I would also plan to be packed and ready to leave from the parking lot. That is, don't assume you're going to go back to your room, check out, have a shower etc. And number three I would try to change as quick as possible and forgo showering until you got to the ship.
I can't remember which year it was or if it has happened both times (we did the dream in 2012 and the fantasy in 2013), but one year there was an accident on the toll road going to Port Canaveral. It was after the point where we could've taken a different road to get to the port and it delayed us by an hour. Thankfully, we are the type of people who are there waiting when boarding actually starts so we were not actually delayed by more than 20 or 30 minutes. The other year we took the non-toll road and the hangup was everyone trying to get to various rental car venues, do your hubby ended up dropping me off at the port and driving back to drop the car off himself. That year was the latest we have ever boarded the ship and I want to say we did not get onto the ship until 12 or 1230. Keep in mind, when we left Disney- Beach club both times it just so happens-it was probably about 730 or eight in the morning. Though we have been able to make it run smoothlyfor other cruises, it just has never worked that way for us when leaving Disney to go to Port Canaveral.
Another thing you might have to factor in is if you have to return a rental car that process does not always go quickly. I would assume that if you were towards the tail end of boarding you would miss the rush, but as again I am an early border I only know the chaos that goes on when thousands upon thousands of people are trying to get on ships at the same time. You have the fact that port Canaveral is not as busy as some of the other ports, and that the megaships don't dock there, but you should still factor in time to return that rental car if that's what you're going to have.
If you were going to park at an off-site parking lot, the same thing applies. You need to factor in time to park your car, get on the shuttle, wait until it's full etc. This may be a situation where if you have a rental car and you think you're going to cut the time close, you may just decide to pay for the car to sit and park it right there at the port. I wouldn't do it, but me being the way I am I would not plan to try to jump on a cruise on race day, that's just me. Too many unknown variables that you cannot control, and any one or two of them falling out of sync has the potential to cause you to miss the ship. As you probably have guessed, I'm never a fly in the day of the cruise person either for all of the same reasons. Lol
Hopefully I was able to give you things to think about and help you with your planning. Good luck!