It's easier on your body.
Treadmills offer better shock absorption, resulting in less stress on your feet and knees, as compared to running on asphalt and concrete surfaces. The extra cushioning and smooth surface also allows for easier runs.You can simulate race courses.
If you live in a very flat area and you're training for a hilly marathon, you can still run hills on the treadmill.You can simulate race conditions.
If you're training for a warm weather race but you live in a cold climate, you can run on a treadmill.
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