An October 2008 revision to the tube map adds a few distance markers to the map. It's 250m between White City on the Central line and Wood lane on the Hammersmith & City. And to the right, 100m between Shepherd's Bush Central line and overland. There's a similar thing at Canary Wharf Jubilee line: it's 200m from Canary Wharf DLR and 150m from Heron Quays DLR. I particularly like the angle they're drawn at - like an elegant version of the walklines I was trying out a long time ago.
But this isn't quite so successful: there's 100m between West Hampstead overland and West Hampstead Jubilee line, and the latter also has a comment that West Hampstead Thameslink is 200m away from the tube. The tube map used to simply show the tube and overland stations as a single node on the map, which I think was better.
Previously: Tube Map with walklines showing when it's quicker to walk. See also Walk the Tube to get healthy, Walkers Tube Map and Adjacent stations.