4.5 MPH Just Won't Do It Anymore

The last few times I have done my interval high-intensity exercise routine, the four minutes at 4.5 MPH just was not enough to build up a decent sweat, and I presume, not enough to get me the cardiovascular benefit that I want.  This evening, on the second high intensity segment, I did 4.6 MPH.  Then, because I felt so good, I added a third segment: 3.0 MPH for eight minutes, then 4.6 MPH for four minutes.  I was sweaty (although not to the point where the cat had to worry about drowning), and I felt great!