Route choice aside, take the miles, throw away the hours, other than as a crude comparison tool. Examine the route profile (however you can manage) and try to classify what part of the route is flat, climbing, congested, etc.
Use your personal performance in prior long rides to judge speed for each type, and do the division.
Keep in mind that, on an unsupported ride, you will spend time looking for food, looking for the loo, looking for a place to stay, and, generally, being lost. You need to figure that an eight hour day will only get you maybe six hours of actual up-to-speed riding in most cases.
Also understand that about halfway through the second day you'll begin to feel every bone in your body hurting and will have to slow down and take more frequent breaks. By the middle of the third day, though, you should have your "sea legs" and be doing a bit better, if you don't beat yourself up or short-change food and sleep.
You're doing 500 miles. Unless you're an exceptionally strong cyclist and the terrain is flat (which I know it isn't) doing that in 5 days is unlikely at best. Doing it in 6 is a maybe, and perhaps what you should plan for, but give yourself the option of taking an extra day (or two), or calling a friend to "rescue" you after 6. And plan at least 24 hours recuperation after the ride.
And don't forget the sunscreen!