The heart is a muscle. Since 1965 it has become increasingly apparent that there is no hiding place in the longer distances for the athlete without good 400m speed. A blatant fact when the last lap of a 10km race is run in sub 53secs. Speed is rate of stride x length of stride. Speed is relative.
Power is required to sprint, the greater the leg power the greater the stride length.
Muscular endurance is required to run longer distances much of this comes from the act of running. More comes if the act of running is made more difficult, e.g. running up hills, running in sand, running with ankle weights, running in water suspended by a buoyant vest. This also improves the heart muscle. Thus, all strength training should be specific to the task of running. One of the advantages of muscular endurance exercises is that the heart is made to work as if running.