The lines vary in length and most represent a good run. The Waterloo & City line is a starter for 10 at only a couple of km, and the longest lines are ultra-marathons in their own right. The Northern and longer I'd split up and do in a few sections.
Lengths of the London Underground lines (from wikipedia)
The lengths are probably listing track length so the run-distance would be a bit longer in most cases.
Should be a fun winter running around London, and it'll make some good reconnaissance for if I ever do decide to have a go at the underground cycling challenge.
P.S.: I found this awesome PDF that details the official colours for the lines and Underground signage, etc. Very interesting!