There are several bridges and long causeways characterizing the unique geography of the Miami area, several of them connecting Miami with Miami Beach. The most interesting and runnable one is Venetian Causeway. The Venetian Islands were built in 1913 and feature gorgeous homes and views. It’s about 3 miles from one side of Biscayne Bay to the other, connecting Miami and Miami Beach. The beginning of the causeway on the Miami side is ~1.5 miles from the heart of downtown.