I've done it on a treadmill once. I just went with whatever speed felt comfortable. I didn't usually keep it at a certain speed the entire time. If I felt I was getting tired, I would bump the speed down to whatever I thought I needed to ensure that I could finish the intervals. If it was too easy, I'd bump it up a little. At this point, there's no need to push yourself to run fast because the the important thing right now is just building time. You can work on running faster once you're comfortable with running for longer periods of time.
Another piece of advice for you is to make sure that you're running at at least a 1.0-1.5 incline. That will help mimic the feel of running on pavement more closely because running outside has more resistance than running on a treadmill because the treadmill actually helps propel you along. Adding an incline will help you exert the same effort as running on pavement.
One last thing to keep in mind is that if you plan on running any 5k road races, you should probably try to get out on the pavement to do some runs to get a feel for it. Don't be surprised if it feels like it's harder to get through runs on pavement than it is on a treadmill.
Good luck with the program - it's a great one!