There is an optimal way to farm xp from the npc that isn't clear at first. Your estimates are about the very high end max gain from a mission, 15 minutes is also about optimal completion. Maybe you are playing the bugged pirate mission spawns right outside the harbor but most take from 5-20 minutes to get to. So lets say 20 minutes sailing 20 minutes mission complete time. Even using your ridiculous low estimates for a new player completion time you are looking at 8 hours.
Even doing nothing but grinding missions the most efficient way to gain xp you could easily be looking at 10 hours to get the next "level" and that doesn't unlock anything interesting besides marginal upgrades/variations of the cutter, and the cutter is free so why risk using a pickle or privateer or whatever.
Playing the small/large battles, random PvP on the map is much less efficient then missions even though they are much more fun. This is one of the main problems with this game and about every complaint/bad review that is a simple fix.