bagrec's just pointed me in the direction of Walk It (basically TFL's journey planner, but for walking places).

It's nice to know that it would take me just over 2 hours at a fast pace (which is what I tend to walk) to get from home to work, which seeing as it's the other side of Zone 4 to Zone 1 isn't bad really...

I think in general, I may be getting up a bit late to consider it for my commute though and may have to stick with the 10 minute train journey for now. However, I have found on a number of occasions that walking around London has been a lot quicker than waiting for a bus or getting on the Tube, so I'll definitely be looking at this again.