The private rooms no doubt look exactly like the mixed rooms except that youre the only one in the room. Its not the HILTON by a long shot, and really the reason you stay at a Hostel is because it is cheap, and there are a ton of people to hit up if you so choose.
As for the SUBWAY system - it takes maybe a day to figure out how it works. You just need that map that has the lines on it - itll tell you which company owns which line, so you know if you need to exit a subway to catch a connection, or whether its a direct transfer. But the Asakusa line is the line that makes the connection with the Keisei Line that takes you smack dab into Narita Airport, and that lets you out right around the corner from the hostel.