Google maps now supports walking directions. The announcement mentions a lot limitations to the service but suggests these limitations are being worked on.
I have yet to get an optimal suggested route, but the initial directions serve as a good starting point, especially since you can drag and drop the path to try out a few alternate routes. From home to work the suggested path was 51 minutes, after tweaking to be more like the path I walk every day the estimate became 48 minutes.
On the subject of limitations, there is no way to specify how fast you walk and consequently all walking times are overestimations for me, e.g. it generally takes me 35 minutes to walk to work; you have to walk around parks and gardens instead of cutting straight through them; there's no knowledge of the Thames Path or other towpaths around London; T intersections don't allow initial travel down the stem; and some nonsensical directions appear.
I think the main limitation is going to be that with driving directions cars travel on named streets in predictable ways, people can walk anywhere in much less predictable ways. I suspect we will not see these directions become "walk along the Cathedral property line, go through the passage obscured between two pillars slightly on your right, follow that path up to Borough High Street, cross the street to Railway Approach, ..."