Look at the skeleton composition. Paragraph 4 supports the thesis in the paragraph Introduction: Almost all sports, whether Indoor or outdoor, require stamina and physical fitness.
Paragraph 1 (thesis):Perhaps the most demanding sport requiring the most heart and lung action
is marathon running.
Body: Although many sports provide very strenuous exercise, they do not utilize all the major
muscles in the way that marathon running does..
Paragraph 2:... muscles used in running ...
Paragraph 3: Although a runner may die from a heart attack, most deaths among runners are
caused by Incautious running, lack of proper training, or not recelving a thorough physical
examination...
Paragraph 4: Another important factor in marathon running is the variety of terrain that adds
even more challenge to the runner's stamina...
Paragraph 5: Most important of all are the statistics of longevity (long life) of marathon runners.
Conclusion
by proving that marathon running is very strenuous
by talking about the terrain a runner encounters