One of the common descriptions is just "I move faster than other people". Or "I see the rest of the world go more slowly". So when they run from point A to point B, it takes them five minutes, but we see it happening in a millisecond.
But there's a problem with this. If "I see the rest of the world go more slowly" is their only superpower, that means, as far as their body is concerned, they're running for 5 minutes. Now, that's not all that amazing - but when you see people running hundreds of miles? Ain't gonna happen. And they'd have to go to sleep every 16 subjective hours or so, too!
Which brings up a very amusing mental image - they're not running across the country, they're just walking across. Speedwalking across. And every few seconds in our time, they have to set up a tent and take a nap, and find a place to grab food from.
In fact, just eating would be massively difficult. You wouldn't ever be able to order anything, because by the time it arrived you'd be long dead of starvation. You'd really just have to forage and steal - you'd just have this traveling node of vanishing food traveling across the country.
Funny, comics never mention *this* part of that superpower.