People will direct you to sites, but a minor explanation (others please amend as required.)
In simple terms. At the equator, One degree E-W = 60 Minutes = 60 NM. But the further towards the poles you go it gets less.
Secondly, if you fly between 2 points you fly a "great circle" route which is the shortest distance on a 3D route.
Look at the sites you are directed at and remember that the world is aglobe and everything is trying to drape their their projections over this in 2D.
Does anyone know a site where they show/display GC lines??