To add one thing. The concrete in the countryside is important as it helps prevent soil runoff which endangers other areas of the environment including the rivers and oceans.

Usually if you see random concrete, only enough is used to accomplish the goal, and it does not cover the entire area. The other alternative would be to completely dig up and replace the soil with a different type of soil...not likely!

I also see hiking as quite popular in Japan. Parks are also very popular in Japan. On weekends it is quite common for families to load up and leave the city for a more nature friendly area.