50 minutes is definitely not enough for a transit....2 hours is minimum (give or take 15 minutes). Narita isn't bad in terms of efficiency (although the airport is a bit dated and I haven't been there in about 8 or 9 years)... If you can, I would pack some sandwiches or keep some of the sandwiches from the flight from Chicago to Tokyo... Narita (and Japan in general) is expensive. Not much you can do about liquids. The bottleneck you will have is Chicago...I've been stuck there for 4 hours the last time I was there...
Take a laptop (or netbook) with you if you can. If you get stuck in Tokyo (unlikely, but if you get really delayed in Chicago, you may miss your flight), then at least you can contact your lady and let her know.
Anyway, good luck, enjoy yourself and remember take lots of pictures... :icon_biggrin: