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The Capital Ring is a surprisingly green route that encircles London. Distance is 78miles; it threads together parks and open spaces, mostly following pleasant residential roads in between. The route is an initiative of the London Walking Forum. The route is very well signed, with a Big Ben logo TQ1678 : Three walk logos, Gallows Bridge. The route is a walker’s equivalent of the North and South Circular roads. Link

Dollis Valley Greenwalk is a 10-mile riverside walk from Mill Hill to the northern fringes of Hampstead Heath linking areas of public open space along the valley of the Dollis Brook, a tributary of the Brent, that rises near Arkley. It passes through Moat Mount Nature Reserve, Barnet Gate Wood, an ice age forest remnant, Totteridge Fields, and under Mill Hill viaduct. It provides many habitats for plants and other wildlife.
Wikipedia: Link

The A406 or the North Circular Road is a 26 mile long road which crosses North London, linking West and East London, from Chiswick to Barking.
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Mutton Brook is a tributary of Dollis Brook and hence the River Brent. It rises in East Finchley and runs about 3km west to join Dollis Brook near Hendon. The Capital Ring and Dollis Valley Greenwalk follows nearly the whole length of the stream.
Wikipedia: Link