Walking on a treadmill you will generally find you travel at 2 mph to 4 mph or 3.2 km/h to 6.4 km/h. How fast this will be for you depends on many factors. If you are tall and have a long stride then a brisk walk will be 4 mph. If you have a small stride that maybe a slow jog. What is important is that you're exercising to where you're breathing heavily.
The average walking speed will be found by collecting your speeds from the workouts you have done and dividing by the number of those sessions. You should find that to progress you need to walk further and/or quicker to get more healthy. We have some workout plans for walkers that will help you lose weight and introduce you to exercising on a treadmill:
When you have completed the walking workouts. Hopefully you may have lost weight and are ready for running. There is a plan we have that involves walking and running called the couch to 5k. This is also done on a treadmill and will have you walking for a few minutes then running. The end of the plan will have you running non stop for 30 minutes in 9 weeks.
The important thing is you get used to regularly going on the treadmill and exercising. You'll find your average speed isn't so important, you'll be measuring your weight loss or how far you get in the workout plans. They are good goals to aim for and getting on the treadmill is just the beginning.