I don't consider Cero Oscuras to be island level but Lanza del Rallempago easily. Cero Oscuras is probably small city large town level since it only destroyed a fair section of Las Noches. Try walking for 3 days. Even assuming you rested half of it. I walk every day and regularly walk over 10 km a day between 2 and 3 hours of walking casually. That's a low ball calculation.