[nanomonkey]

My understanding that the problems occur during (ultra) endurance running style events where proper care isn't taken to keep up with hydration and dietary needs (electrolytes, etc.). Blood can thicken to such a point that the heart starts to get damaged, muscle acidity rises to the point where heart muscle damage occurs, etc.

A fit individual going for a 12 mile or less run probably won't have these problems unless they're in an extreme environment.

To your point, I'm sure you could do similar types of damage from other activities done for long periods of time if the proper hydration isn't followed.