To be considered as running both feet have to be off the ground at least momentarily - hence the increased impact. When walking there is always at least part of one foot on the ground. Race walking rules are very strict abut this.
From Wikepedia:
"There are two rules that govern racewalking.
[4][5] The first dictates that the athlete's back toe cannot leave the ground until the heel of the front foot has touched. Violation of this rule is known as
loss of contact. The second rule requires that the supporting leg must straighten from the point of contact with the ground and remain straightened until the body passes directly over it. These rules are judged by the unaided human eye. Athletes regularly lose contact for a few milliseconds per stride, which can be caught on film, but such a short flight phase is said to be undetectable to the human eye.[
citation needed]
Athletes stay low to the ground by keeping their arms pumping low, close to their hips. If one sees a racewalker's shoulders rising, it may be a sign that the athlete is losing contact with the ground. What appears to be an exaggerated swivel to the hip is, in fact, a full rotation of the pelvis. Athletes aim to move the pelvis forward, and to minimize sideways motion in order to achieve maximum forward propulsion. Speed is achieved by stepping quickly with the aim of rapid turnover. This minimizes the risk of the feet leaving the ground. Strides are short and quick, with pushoff coming forward from the ball of the foot, again to minimize the risk of losing contact with the ground. World-class racewalkers (male and female) can average under four and five minutes per kilometre in a 20-km racewalk."
@kitedoc maybe this is the sport for you?