It really is quite simple to get off the N'Ex go up two escalators to the ticket barrier level, pass through the barrier, and look for signs for Marunouchi North Gate. There is an elevator adjacent to the stairs (a left handed baseball throw from the ticket barrier) if people have luggage. Once outside at street level stand still and look for the taxi rank sign.

There is less walking to a tax from the N'ex platform than there is at say SFO when deplaning from a domestic flight.

From Tokyo Stn. to Midtown is a straight line with no doubling back. Clearly the most efficient way to do it.

Not even remotely difficult. Hot as heck yes, but not hard. :-)