Walt Disney World
The World has several scenic running trails from the Grand Floridian, Polynesian, and Contemporary in the Magic Kingdom area. At the Epcot resorts, you can get your heart rate up along the promenade that circles Crescent Lake past the BoardWalk and Yacht and Beach Club resorts. If you're staying at Port Orleans, you can work up a sweat on nearby trails; Coronado Springs guests run along the resort's 1-mile esplanade.
The roads that snake through Downtown Disney resorts are pleasant, and early in the morning traffic isn't too bad. At the Caribbean Beach Resort, there's a 1½-mile running promenade around Barefoot Bay. Fort Wilderness Campground has a woodsy 2-mile course with numerous exercise stations along the way.