When the mercury drops and daylight starts to seem like a mirage, you may be tempted to take your workout inside. But resist the urge: Research shows that outdoor exercise can boost your mood, your performance, and your calorie burn. (Discover the easiest, healthiest way to lose weight for good, with Run Your Butt Off!) But to reap the rewards, you need to be smart. Here are nine potential pitfalls and how to avoid them.