To start a walk, just click on My Walks on the menu on your Dashboard. This will take you to a section which contains all of the virtual walks which are available.
To find out more about each walk just click on the View Route button below the photo of the walk that you are interested in. If you would like to start that walk just click the Start this Route button.
You will now be able to 'spend' the distances you have walked and added to your 'bank' of steps on that walk.