The difference in distance is due to the fact that there are two options, a traditional one and a modern one that has become more popular. The historic route is marked by ups and downs and it is longer and more diverse. You leave Bilbao through the area of Castrexana past Cruces, Barakaldo and Sestao until you reach Portugalete. The path goes through slopes, forests and urban landscapes.
The alternative route follows the ria of the Nervión. You leave from the Guggenheim and the University of Deusto until you finish the Avenida Lehendakari Aguirre. From there you follow the ria, taking care during the last one kilometre as there is no road shoulder. You have to cross the hanging bridge on a ferry boat for a fair price and you will reach the centre of Portugalete.
This infrastructure was the first iron bridge working as a ferry in the world and has become the most famous image of the town. It was opened in the 19th century and is listed as a UNESCO World Heritage Site. It is also the only bridge of its kind still working.