Yeah I find that faster long runs seem to make everything else seem easier. They're good
It doesnt help that most of my long runs are at Morialta. There is a lot of climb there, so i might have to do some on the flat.
None of mine are fast or flat either, but they are more fun than intervals :)
Too many fast, long runs on the flat might do more harm than good as you're 'pounding the pavement' and not varying your stride much. Hills are our friends. ;o)