Cycling Fitness / Training Advice

I recommend trying to finalise your kit asap so you can familiarise yourself with it.
Also if bivying etc force yourself to to do it in bad weather not just when it’s nice :-)
Sounds like you are riding a lot. I’d also think about cross training and how that might factor in with any previous injuries to prevent reoccurrence.

Generally I think variety of training is best for such events, as much to keep your interest in training.

I'm not sure you're riding enough to include much ultra specific stuff. But the cyclist's training bible is a good place to start.

Be efficient - crappy just spend more money advice coming up - buy a power meter, reasoning being is, if you are going to spend 20/30 hours on the bike a week you might as well make the most of the training.

Weekly periodisation (easy>medium>hard>recovery) and daily periodisation (recovery>hard>easy>hard>easy>hard>easy)

Consistency is key, every day grind. Avoid the 300km Saturday ride and a couple of short rides in the week. It achieves very little

I can manage one hard longer interval session a week, later in the year there would be between 40min and 60min of work at around FTP something like 5x12min with 3min rest or 2x20min with 5 min rest. The rest would be long steady cycling at 75% FTP. Sometimes I can manage 2 of those sessions in a week but I'd be running on fumes and everything else suffers. I also know that for me it is not just the case of make the easier sessions easier to let myself do more harder sessions. I'm yet to be convinced of the value of VO2 work for ultra racing, plus it ruins me and everything else suffers.