South Padre Island, the Sandcastle Capital of the World, is now home to the ultimate Runcation weekend! The SPI Marathon and Half-Marathon begin at the historic Port Isabel lighthouse, crosses the crystal waters of the Laguna Madre, via the 2.5-mile Queen Isabella Causeway, to the sun-soaked shores of South Padre Island. White sand beaches, sandcastles, palm trees and enthusiastic spectators line the route to your finish line at the largest beach bar in Texas!