Hi and welcome.
I visited Japan for two weeks a couple of years ago, so my experience is minimal, but I really thought the food was amazing, so healthy and delicious. There's an abundance of western food (we had cream cheese and strawberry sandwiches every morning for breakfast) and there's Starbucks, hamburgers, pizza - though you might want to give the octopus and seaweed pizza a miss, unless you like seaweed, which I do. I did draw the line at the seaweed donut, though. It was the only thing I ate which I really couldn't stomach. You can also get all kinds of very healthy takeaway meals at the convenience stores, which are everywhere in the cities.
I have to disagree with Octo about the desserts. Red bean paste buns are my all time favourite, and green tea ice cream is fabulous too.