Karefree....you should not be worrying at all what the treadmill tells you right now. You should concentrate on how the exercise feels to you.
This is how the trainer at the gym explained it to me: Use a perceived exertion level on a scale of 1-10 to determine how hard you're working. A 1 would be equal to sitting on the couch watching TV, and a 10 would be equivalent to running from the biggest bear you've ever seen. You want to be able to maintain a 5-8 level for 20 minutes.
Now, this level system is solely based on how YOU perceive your exercise. The girl on the treadmill next to you who's been working out every day for a year might be running at 5 mph and to her, that would be a level 4. For me, running 5 mph would be about an 11!
You also should take your resting heart rate, then multiply it by 1.85 to see what your HIGHEST exercising heart rate should be. Take your heart rate regularly, and if gets above that target number, SLOW DOWN! No sense in giving yourself a heart attack...
I hope I helped.