Does it HAVE to be 600 miles in 6 days? I mean, you can try it, but if you haven't ridden in awhile, you may have a tough time doing consecutive 100 mile days. You could instead shoot for 50 mile days and ride more if the legs feel up to it.

You might want to consider a weekend 'shakedown' tour just to see how you hold up doing 100 miles, that way you can adjust as needed. It's also helpful to find out what kind of gear you need and don't need.