It doesn't really matter what speed you're running at. You might find your speed picks up the more you do but if you try and push yourself to run too fast for too long, you could do yourself some damage. Good for you for doing it in the first place and don't worry about whether you're running too slow or fast. Just do what you are comfortable with.
the best way to start is to run until you get out of breath, then walk until it returns to normal, then run etc....it does not matter on the speed as this will increase, as long as your cardio vascular rate is increased, it will do you good