A simple "longest united flights" search at Yahoo yielded multiple results, including this. Westbound flights will be longer time-wise (generally) than eastbound flights for the same route. City-pair distance is fixed (excluding plate tectonic motion), but flight times and distance for the same route can be variable, depending on winds/ ATC routing. Doesn't seem too difficult a search.

https://thepointsguy.com/news/longes...nited-flights/