Hi Toni!
Thank you for bringing your question to the Disney Parks Moms Panel!
The Star Wars 5K on Friday, April 21, is scheduled to begin at 6:00 a.m. Generally, there are about six corrals for the 5K races that start 3 - 5 minutes apart. The latest you would start would be sometime around 6:40 a.m. You can figure out your finishing time by adding what you estimate it will take you to complete the 3.1 mile course.
I'm not sure what time you need to leave to make it for the cruise, but I would add about 45 minutes to an hour for the time it will take you to pick up your medals, get to your vehicle and then get out of any traffic. It probably wouldn't take that long, but you should add that in it to be safe.
With that, you can estimate your departure time and see if it leaves you long enough to get to Miami!
I hope this helps! Please let me know if you have any other questions.