rTimer Lite is a fine free timer to use for intervals.
You can find rTimer Lite and many more apps that will help you with intervals. Just search for “Interval Timer” and you will see them all. rTimer Lite is a simple free app that you can set your walk/jog interval to be whatever you need and it will tell you, in a computer voice, when to walk and when to jog. If you listen to music while you run like I do it will even interrupt the song you are listening to, which is very annoying. But this is a workout not a music jam so suck it up buttercup. 🙂
To set the interval times, press the Walk or Jog buttons on the app.
You will then be taken to the screen where you can input your interval for that run or walk.
Once you have your intervals input, press the start button and you are off!