Anecdotal, but living in Japan now and I do eat much healthier and walk way more than I ever did. Sometimes it's just for fun since the city I live in is walkable, but also my commute to work involves at least an hour of walking to and from stations which I have gotten used to.
As others have mentioned, social pressure plays a role in fitness, but there definitely is an abundance of unhealthy food. A previous generation may have had less unhealthy food options, so I'd be interested to see if this trend continues. All the greasy fast food chains exist here too and they are always packed.
Funny that you say it, but I think Japan doesn't eat any healthier than the average European country. Coming from the USA, I can see how the conclusion might change.
I come from a country that isn't well-known for its cuisine but live quite long lives, but I still gained (and lost when I left) about 7kg when I lived there.
Nothing about their high-sugar, high-sodium, no-fruits diet seemed particularly healthy.
Okinawans eat very differently than people who live in Tokyo, I guess.