I would like to prepare myself for the climb in the high mountains (for now by "high mountains" I mean elevation 4000 m and more, but eventually eight-thousanders).
What can I do to improve my general physical condition specifically for this purpose (oxygen deficiency)?
I am asking about both types of physical activity, as well as training methods (in the countryside and in the city). The question is only about physical condition (not strength), I've seen similar questions here but they weren't specifically related to dealing with low oxygen levels.