If we don't care about numbers and all that technical stuff, seems like we'd also ignore things like friction like most super heroes. For example, the Flash would incinerate everyone he passes by, including himself unless his secondary power is insane levels of heat resistance, and the moment Superman begins to throw a punch he'd destroy a huge chunk of the surrounding area in a pressure wave far more powerful than a nuclear blast.
That being said, simply giving them secondary defensive powers to allow them to use their own abilities doesn't cut because it would still devestate the surroundings on a city wide scale (and not in the intended way) if we get too powerful. I guess we're just saying "because magic" to ignore other troublesome physics or are we going with something else?
In my multiverse I'd have to say that their magical energy ignores certain mundane physics automatically, so running super fast won't incinerate, create pressure waves, obliterate them on contact with a bug without having to be insanely durable etc. One person also has an infinite speed beam (which some does dodge by anticipating the shot), which would obviously be so powerful it would destroy reality without ignoring mundane physics.
LeeRoy