Answer:
Walking is an exercise
Step-by-step explanation:
The basic idea, no matter how it is presented conveys a simple concept about the paragraph.
In this paragraph, the main idea is presented as an exercise:
Walking is a difficult and safe exercise for beginners. Walking is an easy and safe exercise for beginners. Walking is a boring and dangerous form of exercise. Walking is an exciting and very effective form of exercise
Note the words in bold and underlined. No matter whether exercise is safe or not, the matter is that the walking is an exercise.