Back in the days of PeoplExpress Airlines, the fairly big airline that flew 727, 737, and 747s from EWR, I heard of them offering cheap fares with a lot of backtracking. It could have been SFO-EWR-DEN.

As someone said, the longest commercially plausible route is SEA-MIA. If places outside the continental US are included, maybe HNL-JFK or EWR. GUM (Guam) to NY can be covered by an airplane but unlikely to be an airline route.