We started from Brent Cross tube and walked to Mill Hill East tube via various parks, a 13th Century church, Sunny Hill (magnificent views), Copthall Fields (once the great park of an old mansion), and a disused railway (Copthall Railway walk). The latter has an interesting history - well those of us who like old railways find it interesting.
A tiny bit of history
The Copthall Railway walk was part of the old Finchley Central to Edgware steam train line constructed in 1872. In the 1930s there was a plan to electrify the line and convert to to a tube (part of the Northern Line). However electrification was completed only as far as Mill Hill East before the plan was abandoned in the 1950s. This is why the Northern line has a odd single track one station spur called the Mill Hill East branch. The remaining line continued to be used for goods trains until the 1960s when the tracks were lifted.
And here is a photo of one of its old railway bridges: