One of the reasons walking helps is because it is MOVEMENT. You would/could do the same thing by sitting and rocking on your birthing ball--you are using gravity and movement to get that baby down into your pelvis, the head onto your cervix to start working on opening.
If you aren't in labor, walking is still good to just keep your muscles 'active' and strong and to keep you in moderate shape since labor is hard work!
Having said all of that--our bodies are pretty amazing...and even very sedentary people can have quick labors, can get thru labor, and may even have an 'easy' labor. SO...I say, do what you can, don't overdo anything...listen closely to your body, and let it happen at it's own pace!