The River Thames offers a lot of great walking opportunities, especially on the Southbank in central London, between Westminster Bridge and Tower Bridge. Start at Westminster Tube station, then cross over Westminster Bridge, turn left and follow the South Bank towards Tower Bridge.
Other good areas for walking are along the Thames in Richmond and Kew in West London. You can use the London River Boats for part of the journey, if you wish.
The Regents Canal, a shipping canal from 1816, runs between Limehouse in East London to Little Venice in West London. Angel in Islington is a good place to start. You can then go east to Victoria Park and Docklands or west to Camden, Regents Park and Little Venice.
The Lea Valley Walk (82 kms/50 miles) follows the River Lee / Lea which is a northern tributary to the Thames.