You should be aiming at a fairly slow running pace (at this stage it is more important to get used to the motion of running, rather than worrying about the speed at this stage). Say about 4-5 mph.
Your walking pace should be fairly brisk - about 3-4 mph.
So your running pace will probably be slightly faster than walking, but the difference will not be huge.
The issue is not so much about getting your heart rate high enough, but more because running at extremely slow speeds involves almost unnaturally short steps, and doesn't really develop the running motion and impact enough to start your leg muscles and tendons adapting.