In DS9, the runabouts always travel to and from Bajor at impulse, the trip taking anything from two to six hours. They don't warp even in emergencies.
OTOH, the Defiant
unhesitantly goes to warp immediately after releasing docking clamps in many episodes.
Make of this what you wish. I like to believe in "subspace weather" that is especially bad near Bajor, and in most days makes it too rough for tiny boats to go to warp there. It's easier to warp outwards from the star, or to warp with a bigger and steadier ship. And some other stars may have calmer weather, so that warping in and out of there is easier. (Earth seems like a stormy place, given that the E-D slowed down to impulse there even in an armageddon-level emergency, in "Best of Both Worlds"!)
Impulse speed as such is indeed relatively slow for interplanetary travel, as lamented by Scotty in "Elaan of Troyius" - but still fantastically fast by today's standards. The guesstimate of one day to cross a star system sounds about right.
However, I'd like to insist that not only is "maximum impulse" dependent on the type of ship and engine in question, it's also not a speed at all. Rather, it's a throttle setting.
If you accelerate at 1/4 impulse, or 1/16 impulse, you eventually reach the very same speed as if you accelerated at full impulse. It just takes longer if you use the lower setting, or have a less powerful engine, or a heavier ship.
This essentially means that we can safely ignore all onscreen references to definite impulse settings: all travel times associated with such settings are always correct, because there are too many hidden variables there for us to be able to cry foul.
And we need those hidden variables. Otherwise, Trek would be too full of contradictions on the impulse setting vs. speed issue.