I would be inclined to think that the walking and the 10 minutes other workout should be enough on a daily basis. Rather than just abs every day, why not mix it up so that you are also working other areas of your body as well. Our arms, legs and other muscles also need to be factored in. Good for you for walking that extra distance to/from work :-)
For me, the distance walked isn't as important as the time you walked, and waked it reasonably briskly.