This is the other great waterside running spot in Lisbon, along the Tagus River from the Marina to Rio Trancão, which is 8.7 km out and back. The flat route has great views of the water, and a dedicated path most of the way. A highlight is the run under the Vasco da Gama Bridge, which opened in 1998 and at 17km (11 miles) long (10km/6 miles of which pass over water), is the longest bridge in Europe and one of the longest in the world.
A good spot to start is the marina, next to the Pavilhão do Conhecimento (Pavillion of Knowledge), and follow the waterside path east. The path passes through the Parque das Nações, the Oceanarium, and the Jardim do Passeio dos Heróis, before going under the bridge. The path continues for 1 km or so past the bridge, to the end.