I still think New Castle, VA has the record for 1000 K's in one day at one site, something like 17? This is like 20 years ago or so, covered a couple times in Soaring magazine (the day and then a reunion). Some even flew it dry, sorta Iron Man a dry stiff winger. Regardless, a long flight, even in good conditions.
My longest glider flight is "only" 7 hours. Done on thermals in the NE US.