For a nice long flat walk beside the Vltava river, passing through Prague, consider some (or all) of the 30 km A2 cycle path starting near Zbraslav and finishing at Troja zoo.
Tag: Braník
A short walk along the Vltava cycle path between Braník and Modřany: 4 km (extendable)
The A2 cycle path along the Vltava which runs through Prague is also a good hiking route! For a short walk try the section between Braník and Modřany (extendable into Modřanská rokle).