You're supposed to run 3 days a week. On the 4 days you don't run, you are supposed to walk 2 or 3 of those days. The remaining day(s) are rest periods. For example, I run on Mondays, Wednesdays, and Saturdays. My walk days are Tuesdays for sure, and Fridays or Saturdays maybe. My rest day is Sunday.
As to how fast you run when you run, run only as fast as you can. I have heard that you should be able to speak at least in short sentences when you run, but that is up to you. And if you feel like you can't run for the full minute or however long it tells you to run, then you should start walking. The point is that you know your body and you know your limits, or at least you will learn them while you're doing this.
Yes, it is essentially a timer, but it is also an encouragement. I find it incredibly helpful to have a voice telling me, okay now walk for a couple of minutes, or run for a minute. Also when she says that I'm halfway done that's also an encouragement. Without this app I think I would have given up by now.
Also, it keeps track of how many miles you run. All you need to do is turn GPS on before you start your run, and at the end of the run it will tell you how far you have gone.