Search for directions as usual, by default driving directions are displayed. Notice that there's a new link titled "Walking" (see screen shot). All you need to do is click it. If you'd prefer to see this in action, see this screencast (web slang for screen recording)
Tip: The quickest way to get directions in Google maps is to use the form "start to end". For example: london eye to westminster abbey. This goes for postcodes too. Eg. NW1 to NW2
Of course, for those living in the UK, this was always possible with Streetmaps. But with increasing number of people choosing Google over the previous favourite Multimaps, this feature will only help popularize it even more.
See also: Bus Stops and Timings with Google Maps