I often average around 450miles (mostly hwy), 720km.
My best though was a round trip from Toronto to Ottawa and back. Almost all hwy @ 80km/h (50mph). About 887 km (550miles). That was driving like a granny on a mostly two lane highway, so if you got stuck behind someone you were likely there for awhile.
600 miles would be difficult.. but doable with some good hypermiling and likely some slight eco mods (newmission + LRR tires etc..)