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The Parkland Walk is a 3 mile linear green pedestrian and cycle route in London, which follows the course of the railway line that used to run between Finsbury Park and Alexandra Palace, through Stroud Green, Crouch End, Highgate and Muswell Hill. The route is shared with The Capital Ring.
Wikipedia: Link

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