you won't need to measure the running "track" - this is usually known by the crew or fitness staff. If you run in the morning, it shouldn't be crowded, the hard part is keeping track of your laps. Instead of counting your laps, you should probably just run for time. When I'm on a cruise, sometimes I'll alternate between "running track" and treadmill. It's not a big deal, I've put in great training while on a cruise.