That's rolling.I was final able to hold 4.5 mph for a kilometer. To hold above 4.2ish I end up in a very odd trot that is almost a run, but both feet stay on the ground and I don't bounce. This pushes my HR to just below lactate threshold in the 170s, which is higher than most of my runs.
Definitely gives me a lot of respect for the physical fitness of WWII dogfaces and the old breed.
Now you're ready for the Race walking olympics. If you haven't seen that it's worth a google.