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If you can't (or prefer not to) run outside, you can train for a marathon on a treadmill. Note: this is not recommend for long training runs of 10 miles or more (although runners do run that far on treadmills). Treadmills offer a more forgiving surface than asphalt or concrete, and some runners find them a pleasant change from harder surfaces, and a more appealing option in the winter. But treadmill running does take some getting used to; be careful when starting and stopping your run so you don't fall, and make sure that your shoelaces are not dragging. And if you notice any leg or foot pain after a few treadmill runs, go back outside or try a different treadmill. A cheap model may not have a surface that feels good after 5-6 miles of running.
Want a hill workout? Most treadmills can be adjusted to various degrees of incline to make the run more challenging.
Running a treadmill is a great alternative (inside), especially if you live in an area that has bad weather; however, I would not always recommend it, because running outside gives you the ultimate exercise of using most of your other parts of the body (i.e. arms). Plus, you are also using your own weight, whereas a treadmill you might be able to cheat a little bit.
Excellent point! Personally, I love running outside, and the only times that drive me to the treadmill are really bad weather (such as icy roads) and business travel, when I'm in a location where it isn't feasible for me to safely run outside alone in the early morning.