You should be picking up the speed now. Make sure to write down the speed levels on your apparatus. Note how long you can keep up a sprint at a certain speed, then try to do better next time.
If you're sprinting along and you can't keep the speed up, turn it down a bit to finish your minute run. That run is the most important part of HIIT training. It's in that minute where challenges are created and overcome. Kick the speed up a notch, adjust, and push yourself harder in that time span. You'll feel rewarded when you get to walk and catch your breath.