So one way to think about this is that running and walking will both get you to your baseline cardio fitness goals, but running will just do so faster. Right now, the American Heart Association (AHA) recommends that adults get at least 150 minutes per week of moderate-intensity aerobic activity (like walking), or 75 minutes per week of vigorous exercise (like running or HIIT)—or some combination of the two—spread throughout the week, as well as two days of strength training… More here.
Interesting stuff. The thing about walking is that it is open to many more people than running and is almost always the best exercise to start with if you want to improve your fitness.
I agre with Basically when we walk we have trained the heart. Train to keep our hearts healthy. it sounds trivial but in fact it is very useful for our health