This site is dedicated to the north west of London - simply because that is the area that I know best. What about walking by rail further afield? There are, of course, many more opportunities for walking around London on sites like Saturday Walkers Club and Out of the Loop, as well as across the country with Car Free Walks.
It is always exciting to find a new walk in the Chilterns though. So thanks to Stephen Mitchell who recommends a “gorgeous” walk up to the Sharpenhoe Clappers. That is an area of ancient woodland and chalk escarpment north of Luton. Much of the land is owned by the National Trust.
Chilterns Area of Outstanding Natural Beauty have produced instructions for 9km and 11km circular walks in the area, starting from Harlington station:
The Saturday Walkers Club also has a walk from Leagrave station to Harlington which has an option of walking through the hills.
Harlington station is on the Thameslink line to Bedford. It is a 45 minute journey from St Pancras and being on the Thameslink line, there are direct services to all kinds of places, such as East Croydon, London Bridge, Farringdon and West Hampstead. The service looks good, with four trains per hour all week.
Stephen also suggests walking from Hitchin station up into the Pegsdon Hills. Again the Saturday Walkers Club has instructions for doing that in their 25.6km Hitchin Circular walk. That walk also includes an option of going over to Harlington station (about 16km from Hitchin).
Hitchin station is also on Thameslink, although a different branch (trains to Peterborough or Cambridge). There are three or four trains per hour and a direct service from Finsbury Park as well as the central London Thameslink stations.
Thanks to Stephen for these suggestions. I am looking forward to trying them!
