Walking meditation is often used in vipassanā meditation, especially during retreats. In this book Ajahn Nyanadhammo explains various techniques of walking meditation.
If you practice samatha meditation on buddho, walking meditation is not directly part of the meditationsystem itself. Ahba teaches that it is extremely difficult to achieve very high concentration while walking and having your eyes open.
However, it is good to keep repeating buddho mentally, during a retreat, but actually at any time during the day. This is a form of sati (mindfulness) with which you protect the mind and if you practice like this it also becomes more ‘normal’ for the mind to focus on buddho. This greatly benefits the concentration process.
In that sense, walking meditation is also a valuable addition for the practitioner of samatha meditation on buddho.
A nice detail is that Ahba is sometimes invited by Ajahn Nyanadhammo to give his blessing.