There is a lot of prejudice in Japan against 'older' workers. When I was looking for jobs teaching English, I was told over and over again that I was too old, and that they wanted someone under 30, or under 35, or whatever.

The problem with getting any other type of job is the language barrier. Unlike many countries in Europe, where everyone speaks English and it may be used frequently at work, the number of people who actually speak English is relatively low, in spite of everyone being required to study it for six years. I think things have improved in the language department, but not to the extent that you could assume that more than a few people in a company would speak English well enough to work easily with you.

The only people I met when I was there (admittedly long ago) who did other kinds of work had all been transferred by major companies or spoke Japanese.

English teaching really is your best bet, unless you have a special skill that is in high enough demand that your lack of Japanese, and possible need for a translator, can be overlooked.