n00b question - am I missing something?

So I just started my first career.
Mission 1: Head to South China Sea - Check
Engage merchant ships - Check
Head to East China Sea - Check
Patrol East China Sea - Incomplete?

So I've been in the East China Sea for a few weeks now, maybe even a month. I've sunk about 30 enemy ships, close to 200,000 tons, and i've been back to Midway twice to re arm. How much longer do I need to patrol the East China Sea for it to complete? Or am I not doing something right?