A good bet for an 8 hour eating spree would be in/around a major bus/train terminal. Tokyo Station, Shibuya, Shinjuku, etc…. have many of the casual eats you want in a concentrated radius.
In addition, the streets immediately funneling into the stations are packed with all kinds of eateries as well, catering to salarymen/women and travelers.
Japan covered shopping streets are great for food also.