This is an older study. I have learned that just because a study is old does not mean its finding is not relevant. Indeed, since newer studies focus more on pharmaceuticals than lifestyle, much can be gleaned from the oldies:
Walking. A Method For Rapid Improvement Of Physical Fitness, Journal of the American Medical Association, May 1980
This study shows that it is possible to improve substantially aerobic physical fitness in three weeks by walking daily with a light backpack load. This program is most useful for people who have low initial aerobic work capacity.
Walking is safe and comparatively harmless. Moreover, it can easily become a way of life and be recommended as a popular method of physical activity, even for elderly people.
And this …
In our study, the main determinant of the rapid increase in VO2max was not the speed or duration of walking, but the increase of the weight of the backpack load.
I must increase my aerobic work capacity in winter for the extra weight I carry in clothes. Boy do I pack it on!
By the way, I did a search on Google for this study, “Walking. A Method For Rapid Improvement Of Physical Fitness” and it did not find it. At all. Yet, it was the top return when I searched for it on Duck Duck Go. This isn’t the first time I couldn’t find what I was looking for on Google. I depend on them less these days. Something is going on with their search engine.
I have switched to Duck Duck Go as my primary search engine. Google Scholar is also useful when searching for academic papers.