The speed doesn't matter, IMHO. What matters is your perceived level of intensity, and getting your heart-rate up.
What's fast enough for
me to feel challenged will be too slow for others. I read all the time on 3FC about how people are going "slow" at 5.0, or more! I started at 3.5, and have only recently gotten up to 4.0 mph. For me, that's a nice jogging pace. For others, it's a warm-up walk.
Anyway, on a scale of 1-10, with 1 being sitting on the couch and watching TV, and 10 being "my heart is going to explode out of my chest and I'm about to pass out because I can't breathe!" you should aim for a 5-7.
As for your target heart, that is unique to you. There are websites out there that can help you figure out what it should be.
Hope this is helpful to you.