There is minimum level needed to survive and different higher levels needed to thrive and succeed.
You can survive without any Japanese as a tourist. Japanese are unfailingly polite and helpful, even if their attempts at English are difficult to understand. Japan ranks 127th in the world in English fluency, just above North Korea, so don't expect everyone to understand English.
After taking a trip to Japan, you'll be grateful that romaji (roman letters) are available for all train stations and the government makes many things available in English and other languages; websites, pamphlets, maps, immigration and customs information... almost all services. You'll be thankful Japan has yet to catch the "English only " mania in the US and make everything only available in Japanese. I knew an American here who ranted about no Spanish or other languages should be used in the US, then ranted because English wasn't on every menu for him.
Remember that languages are almost half idiom and culture-based slang. You're not learning those, so you would not understand many of the things being said from your more formal study of the language. But don't let that stop you.
