Frequent runner here and if this is a make or break thing for you, it might be better to consider another line or itinerary. Having done cruises even during different marathon training cycles, I'll share there is not a great opportunity or route on CoCo for a run.
We have been on the private island for
DCL, Royal (Coco and Labadee), MSC, and Holland (Carnival's place) and really only Disney has enough space/path that takes you away from other guests that you can really get a good run in. The others do have some areas that you could get a run in but you are more likely too close to other guests and will be zigzagging or interrupting your pace. CoCo might work but there is little paved areas and you would have to incorporate more soft, sandy areas. Holland/Carnival has a decent out and back stretch to the horseback area and another place that can work but again the terrain might not be as great, there is even a a single track but it is even less suitable for running.
All that focuses on running on private islands. I'll add that the ship or itinerary can be a great place to run. Holland does a thing that they have a group walk/run for a cause. It is more of a walk and it takes place on the promenade deck but nice to actually do something with a group and support a cause, (I think our event on the ship went to a Ukraine relief group). If you are considering Royal, the Oasis class is one of the best to do runs on the ship. Since they are so large and the run/walk track is around deck 5 so it only take 2.5 laps per mile.
It is so much better getting less laps per mile, but it can get a little crowded at peak times in the mid to late morning.
As far as itinerary goes, getting off the ship and going for a run is a great way to explore. There might be a few ports that you opt not to run at but most, if you have even a little bit of a plan (thanks to the GPS map apps/sites) you can be relatively successful. I have done 5k up to half distances off the ship. Curacao, Reykjavik, Kristiansand were some fantastic places to see on foot.
