http://rodcorp.typepad.com/photos/art_2003/tube_walklines_final_lm.html

For some journeys it's really not worth getting on the tube: it takes a long time, and costs you money. Sometimes it's quicker and easier to walk.

This is a slightly altered map showing which stations are an arbitrary and as-the-crow-flies 500 metres apart from each other. The "walklines" are dotted in.