First of all, I'm 67 and don't do anything special to stay healthy. I don't worry about hydration(more hydration = more piss stops). I eat a big breakfast at 7am and maybe have lunch around 2pm. Supper is after I stop for the day, usually around 8-9pm.
Fact of life is that the length of time you can travel decreases from the beginning of the trip. Second fact of life is that often times the ergonomics of the bike are the limiting factor of your daily saddle time.
For me, max comfortable time is around 12-13 hours. With gas, meal, and restroom stops, I can comfortably do 750 miles for the first two days. After that, usable travel time drops to about 10 hours per day. This is when I want to get somewhere and am using the interstate, cruising at an overall average of 70-75 mph.
If I am somewhere scenic I'll take the slower roads with better scenery and riding(twisties). This will drop my daily average speed to 25-30 mph or less for daily totals of 250-300 miles.
My suggestion for preparation is to make sure you can sit on your bike for hours on end without setting fire to your butt. If you can't sit on your bike for 12 hours, all the rest of your preparation is meaningless.
My understanding is that you have a lot of ferry time the first day or two which will necessarily reduce the mileage you can cover by reducing saddle time. The plus side is that the ferry will reduce saddle time for the first days, making you fresher for the following days.
If you plan 12 hours of travel time for the first two days and then 9-10 hours for the last two I would think you will have no problem. I don't know the speed limits you'll be riding in, but a 60-65 mph average should be doable once you get past the ferries. 300-350 miles per day the first two and 600 per day for the last two shouldn't be too hard to do. Of course, the more miles you can cover the first two days will make tha last two that much easier.
Don't make hotel reservations ahead of time. The most dangerous thing you can do is to have to push to get to a hotel where you have a reservation. Look for a place to stay when you are tired and are getting close to where you can't go on. Start early with the first light. I find that it is very easy to cover a lot of miles before noon. After that, it seems the traffic gets a lot harder and more tiring to deal with.