How do you add walking services day by day basis if you have a pending overnight stay?

I think the easiest way to handle it would be to have the owner request another stay for the time period spanning when she needs walks. Then you can adjust your nightly rate down to $0 and itemize the walks to show how many are being provided over that time period. This ensures the dog is covered under Rover's insurance during that time and that both you and the client are clear on what's being provided.