I am not a Green (with a capital G) walking and alternative transport evangelist, but I do admit that I try to walk when I can.
Long before I ever started the SE23 Challenge I decided that I would never stand still on an escalator or take a lift if I could walk up stairs. If you are a commuter, the best free "extra" exercise you can get is to walk up the escalator no matter how tired you are (allowances for baggage can be made). I noticed an enormous difference in my stamina in only a few months.
Having said that, Richard over at Baggage Reclaim mentioned a site I had not heard of called WalkIt.
The idea of the site, and the group, is that you could consider walking your intended route across London, so it offers directions as well as a measure of distance, time required and calories burned! Excellent!
Of course this is not reasonable for all journeys (I work from home now but I checked my old commute and it would take me 196 minutes at a reasonable pace each way) but if you have a meeting to go to, tell the boss you are off, then walk it instead next time. It isn't a lie, and if challenged, tell them you are investing the time in strategic thinking, your health and in minimising the company carbon footprint! Dare them to challenge THAT!
I have added the widget to my site although I think it will need a little more work on their side (it will show you the route from your postcode to Forest Hill station), but if it inspires you to walk a little more, let me know.