I am sure your question was answered by now but I also had this problem and had to contact Rover support so I thought I was post my solution here in case anyone else had the same problem.

I did not have "Today" as a tab either on my app, despite having the most up to date version. My only tabs were: inbox, rebook, services, your pets and more. I was informed by Rover support to uninstall and install the app. When I did this and logged back in I had the "Today" tab and was able to track walks. Hope this helps some people.