Lots of zombies in the way, and a very long round trip run.
1. Enter via the very southwest sewer entrance in Moresby (The one you escort the guy to in the storyline quest) 2. Run Straight until you hit a wall and there are paths on either side of you. Go Left. 3. The path is fairly linear from there, there are a couple rooms which widen out but they all end up converging back into the same tunnel. Ultimately leads you to City Hall. 4. Travel through City Hall to the other sewer entrance next to the dead guard's body. (can get a bit confusing in City Hall but the door you are looking for to get to the sewers isn't the one next to the work bench.) 5. Once in the sewers connecting the City Hall with the Market there are no paths leading anywhere other than the two destinations so you cannot get lost in this set of sewer.
To Leave: 1. Set your quest tracker to track any quest, it will set the fastest route back to the sewers you had entered from in Moresby. 2. Run!