If you just started riding or haven't ridden in a long time, then it might take a while for your butt to get used to it.
If what you are feeling seems like it's down in the bones, then just be patient for two to three weeks. Ride as much as you can stand.
If the person that owned the bike before you rode a lot and for long rides, it probably has a decent enough saddle. If they only rode short rides and infrequently, then it might be a terrible saddle.