A couple of thoughts.
Don't expect to do what you can do for one day for a series of days: rule of thumb is that doing xxx km in a day means you can probably do around 75% of that amount per day for successive days. A rider who can do 400km for successive days would need to be able to do, say 550km on the first day. If 420km is your maximum for a single day, you might manage 320km per day on an ongoing basis. Quite a lot of people who have done 400s in well under 19 hours have not been able to do more than 300km per day for a couple of weeks (me included).
For training, you would need to do something along the lines of lots of steady base miles + a long ride at the weekend + two interval sessions per week. Look at Trainerroad, Zwift, etc, for a more palatable way to package it. But you need to be able to find a way to fit it in with your commuting. I can only think of two or three people on this forum who could do/ have done >400km for multiple days. One of them is Wheels of Fire, who - when talking about his LEJoG record - said that the core of his training was 45 miles per day commuting, plus two 2-hour turbo sessions at very high wattages, per week.
You should include some speed work as you need to be as fast as possible to get time to sleep, eat, etc.
If you can do the kind of daily distances you are targeting it would be a shame not to test yourself competitively and do some long time trials: 100miles, 12 and 24 hours. They would be excellent training rides and allow you to compare yourself to top riders, and get some great advice. And / or some ultra-races: 400km per day would put you amongst the favourites for the TCR.