I haven't lived in Manhattan for almost thirty years but I still have family and visit frequently. A few thoughts:

Your character will need really good shoes and be in shape to do a lot of walking -- subways and surface transit will go down almost immediately. A bicycle is better.

Getting to the mainland from midtown or downtown would be challenging on foot; I think that the Third Avenue Bridge would be my top choice.

Historically New Yorkers have done very well coming together in crisis situations -- I suggest that your story have compelling reasons on why that isn't happening.