What is the best way to train for a 5 day, 500 mile, bicycle Journey?

That will be quite an accomplishment but completely doable. Hopefully you are in an area where you can get out and ride your bicycle safely. If so I would say work yourself up slowly either on the road or on a stationary bike at the gym or home. When I was a teen I would ride 45 miles to the beach and back stopping for traffic lights and spending several hours at the beach. I had to travel through a lot of cities in Southern California. It took about 5 hours of riding. Once I went 87 miles to a further Beach and it took me 9 hours of peddling but there were a lot more hills and we had a couple of flat tires. That was in the 60's and I had a 10 speed. I am in my mid 50's now and would have no problem riding 10 miles in a day but my knees are worn out. You are going to need to have good strong knees and leg muscles. You don't want to overdo it. So take it slow and gradually build up those muscles. Ride 2 or 3 times per week giving your muscles time to relax in between.

Just try to ride every day at least a couple of hours, with a long 6-7 hour ride on the weekend. If your route is hilly, work some climbing into your training rides. When the day comes, make sure your bike is in good condition, have plenty of spare tubes, patch kit, basic tools. Have enough clothes to be ready for any weather, no matter what the forecast is. Have fun! I wish it was me going!

Ride lots. Find Bicycling magazine's issue on training for a fast century. They lay out a several month program similar to a running marathon build up. My guess is that if you are in shape enough to ride a fast century by then, then you'll probably be able to make the multi-day ride at a lighter pace without too much problem. Good luck.

You must stay committed to it. You go backwards strengthwise if you are off the bike for even a week. That's a crazy long bike ride and it is going to take you the entire 6 months of hardcore riding to work up to it.

Start riding regularly, working your mileage up gradually.

Get to the point where you can ride 60-80 miles on consecutive days comfortably.

Most importantly, make sure you have the bike set up correctly before you start or you will fry your knees/back etc.

try driving ur bike for 3 days in a row for seven hours a day.