That was when the Spartan 2s were 14 years old, and had not adjusted to their augmentations. We know in real life the top speed of a normal human is 40 mph. http://m.livescience.com/8039-humans-run-40-mph-theory.html So even without their armor they would be able to run it speeds greater than this as they are superhuman. Also I want to address that when Chief tore his tendon when he ran 65 mph, was bs. We know that Spartans can move 5 times that of their normal top speed without issue. Halo: The Fall of Reach pg. 115 said: "More importantly," Dr. Halsey said, "the armor's inner structure is composed of a new reactive metal liquid crystal. It is amorphous, yet fractally scales and amplifies force. In simplified terms, the armor doubles the wearer's strength, and enhances the reaction time of a normal human by a factor of 5" So the fact that he tore his tendon by not even running twice as fast as a peak human is absurd.