Still there run speed should be a little bit higher.
And most medieval Knight horses were "heavy horses", full of muscles, not those used in the steppes. "Brabants trekpaard" was a very famous one, and used by many countries in the middle ages.
These horses are still in use in the forrests and agriculture in Belgium.

Here is a nice picture:

http://www.monument.vlaanderen.be/vc...ard/fotoa7.jpg


Still, I think there running speed should be higher, but faster fatigue when running.