There is some lovely seaside running just north of Gibraltar in the Spanish town of La Línea de la Concepción. On the east side, there’s a seaside path along Calle Torrijos/Paseo del Mediterráneo heading north for 3.3 km one-way to Puerto de la Atunara. There are great views of the Alboran Sea. The wide beach (Playa de Santa Bárbara) along this road is also good for running, with fairly firm sand. The views of The Rock heading south along these paths are spectacular.
On the western side, it’s equally enjoyable to run along Av. Príncipe de Asturias (CA-34). The palm-lined road has a path along the water for 3.7 km, north to the shipyard. It’s a busier road than on the east side, but the path is excellent and the views are nice. Note both of these paths are very open and it can be very hot in summer.
It’s easy to run between the east and west sides, especially at the southern end. It’s 1.5 km, using the pretty Canino Princesa Sofia Park. So if you do both sides plus the park, it’s 15 km (9.3 miles).