Not sure where you are getting your wattage info from. Exercise bikes can be dreadfully terrible at giving good estimates on that. The exercise bike I use at the gym this year (they have some new bikes this year) told me yesterday that I was doing 150 watts for much of the hour that I was on the bike. I am 71 years young, I really wish that I could do 150 watts that easily, but no, it is not going to happen. Their older bikes usually had me struggling to maintain 120 watts.
The only wattage data that I trust is based on timed hill climbing. I am an engineer (retired), thus the math to calculate the watts needed to lift the weight of me and my bike X number of feet in Y minutes is based on formulae that I worked with in college, assuming no friction losses, etc. Thus, I have calculated wattage from actual riding up hills, and I know with certainty that the exercise bikes at the gym can't be trusted. And stairmasters at the gym use an assumed average person weight for calculating wattage, those can't be trusted either, unless you are able to enter your actual weight into the machine. I have calculated my personal wattage on a stairmaster the same way as on a bike using steps per minute and height of a step to calculate how many feet I climbed in an hour using my actual body weight.
Given your past touring experience, especially your UK trip, you probably have this figured out pretty well already. That said, too much focus on up to an hour of training every day or every other day is good for muscles and cardio, but it is not doing any endurance training to be able to fuel your muscles after your initial glucose stores are consumed. I think during your final month of training that you need to do at least a two hour continuous training session weekly to train your other organs for providing the fuel you need for endurance. Maybe a week before the trip try to do a four hour training session, but get off the bike for five minutes every hour if you do that. Since this is to train your internal organs, you do not want to be adding lots of carbs during this training. When I try to train in the spring before a brevet, I try to do at least three to five hours before I have any food, only have water or coffee (black). I want my body to be able to function on internal fat reserves for fuel during this training.
Example, two years ago a friend of mine that I have done several tours with before and I were planning to do Natchez Trace in early April. During winter, I was doing hour long bike sessions at the gym three times a week. Sometimes did stair master instead of bike. Natchez Trace is roughly 440 miles. Our longest planned day was 60 some miles. But I also planned to ride a 200k brevet on April 1 a few days before we left for Natchez Trace. So, I was out in February and March as soon as most of the snow and ice had melted (in southern Wisconsin) building up distance and continuous hours on the bike. In February I was doing 20 some mile long rides, late March I had increased that to 50 some mile rides, trying to find ice free places to ride. I usually try to do at least a 90 mile ride a week before I do a 200k brevet, but that did not happen this year.
Photo below was from one of my training rides, mid February, unfortunately this part of this trail had not been plowed, it was solid ice.
As April 1 approached, the forecasts were terrible for the day of that 200k brevet, so I canceled my plans for that as I was sure that I would DNF it. Forecast for the day of the brevet was for temp in the low 40s (F), winds in the 20s (MPH), and a bit of drizzle or snow, and the forecasts were accurate. I was glad I did not try to ride it, especially since my longest training ride was less than half the distance of a 200k.
But, doing a few 50 mile rides trying to prep for the brevet meant that I had forced my internal organs to function well to deliver the fuel I needed to ride past that first hour or two when you are burning stored glucose. Thus, I was ready for Natchez Trace. But, the friend of mine chose to just do interval training on a bike on a trainer, no training sessions were over an hour. When we started riding Natchez Trace in early April, during the first hour I had trouble keeping up with him. After two hours, we were both riding at about he same level of effort. After three hours, he was toast, but I was warmed up and ready to keep going for hours. My point is that you need some longer sessions than just an hour.
You have done a lot of touring before and if my memory is correct you had high distance days on your UK ride, so I think you will do fine regardless of how you train. But, given your route, I think you will have some zero days due to weather. If you can get ahead of schedule early on, that could provide the buffer you need for a few zero days.
And take a look at your route, how hilly is it? My bike tour last summer was flatter than most of my previous tours. I only had one day with a lot of nasty hills, my median day was 65.2 miles(104.9km), average was 57.3 miles (92.2km) and I had two days over 80 miles. But, this trip was quite unusually flat, my typical tours have much shorter distances. And I load my bike heavier than you load yours, you probably would have been able to do much longer distances than me. Photo below as from my tour last summer, this photo was about half way through one of my 80+ mile days.
