Google Maps already supports
driving directions for users in
Africa, but for those without a
vehicle the service has been
updated with a very useful new
feature — walking directions.
Starting today walking directions
will be available in 44 African
countries, covering a sizeable
chunk of the continent. Of course,
as with all of Google Maps, the
results aren't perfect, so there may
be a few issues with the directions
when it comes to pedestrian
crossing and sidewalks. The feature
is currently live so you can check it
out now on both the web end
mobile versions of Maps.
Source Google latlong blog

Nice, but it probably won’t work in parts of Nigeria where they build anyhow.

Why won't it surely will in some reg parts

Because, most of the walkways aren't defined, it's only in the bigger cities you find genuine ones. In the smaller cities, it will be more of a bush path, which will vary with each season.