The best running option on the south coast is a 2.5 km stretch of oceanside road in Atlantic Shores. At Miami Beach, Enterprise Coast Rd. heads south for 1 km. Then, bear right on Seaside Drive, staying along the water for another 1.5 km, passing the South Point Lighthouse at the 2 km mark. There are decent water views in some sections, houses in others, but this is a relatively quiet road and the best running in the area.
For a longer run, there are some quieter side streets in Atlantic Shores inland from the water where you can rack up some mileage while not being too worried about cars.