When it comes to temperature, fall running can't be beat. After months of heat and humidity, your legs will have some extra pep in them on those first crisp days. And since it's a little cooler, you'll no longer become drenched in sweat three steps into the first mile. In fact, breaking a sweat might actually warm you up a bit. The shorts can stay on for a little while longer, but the fall season means you can trade in your running tank tops for cozy, long-sleeve shirts and zip-ups.