I had serious trouble with harpies blocking my path when I try to go back down north so I devised this alternate strategy:
Step 1: Do not flee with the Titan away from the outpost, instead demolish all of the buildings in the back of the settlement so the harpies won't go back there, remove any soldiers/buildings that would lead infected to the back.
Step 2: wait until the harpies pass, after that a swarm of fast infected attack, but its easy to avoid them with the titan, then demolish a whole in the wall and go around that hoard to the south
Step 3: Follow the hidden path west of the southern pass. Then start running like hell, because you only got so much oil.
From there just follow the path as normal, if you are fast you can be finished with 6 oil to spare. I hope this helps someone who was having the same issue I was having.