The Thames Valley Trail follows the Thames and North Thames rivers for 109 km, giving London an easy way into its river valley on foot. You do not need to tackle the whole route: the paved sections through Gibbons, Harris, and Springbank parks are good for a relaxed walk.
For something longer, pick a stretch and arrange transport back. Local hiking groups also use the trail, which can be a useful way to see the less obvious green side of the city.