Lets not beat around the bush, the 312 is a long day even for a well seasoned cyclist so for your average amateur its going to require some serious staying power.

The key to ensuring you get round and also enjoy the experience is to focus on the following.
⦁ Consistent training, so riding your bike at least 5 days a week, ideally 6.
⦁ Accumulate in excess of 10+ hours during that week, ideally some weeks pushing that out to 14-16 hours.
⦁ You also need to do some long rides, one to train your body, not just the cardiovascular and muscular systems but also your neurological system to tolerate the duration you will be riding for.
⦁ And secondly you need to work on your ability to consume and utilise sufficient fuel (carbs) to complete the event. I can't stress how important this aspect of your training is to ensure you are able to complete this without getting swept up by the broom wagon.

Given the intensity you'll be riding at the vast majority of your training should be below the lactate threshold or your FTP.
That's not to say you shouldn't do some intensity but the primary focus should be on working mostly aerobically, therefore we need to build the largest aerobic capacity that we can. And the way to do that in a consistently repeatable way is to keep it steady.

I see too many people sign up for these events, go from 5 hours a week training to double that at an intensity that is way too hard only to then get tired, ill and then lose motivation. This leads to missed sessions and a lack of consistency and it's this word - "consistency" that will allow you to get the most out of yourself. Don't overly focus on the goal, ideally focus on the process and the goal will just happen.

So order of priority

1. Consistency/Frequency
2. Volume/total hours
3. Intensity/prescription

I'm not saying the 3rd one isn't a priority but it shouldn't be at the expense of the other two. As an example if you can't hold the power as prescribed during a sweet spot session, don't just bin it off (unless you are ill) rather just turn down the intensity to a level you can sustain so that you don't sacrifice the other two.

The intensity you ride your long rides in your training will ultimately be the upper ceiling you look to hold during the event.
So if you struggle to do a 4 hour ride and average 200w, then come event day whatever you do don't start the ride above that, this is the first big tip. I know that might sound blindingly obvious but trust me I've seen many a good cyclist get caught up in the occasion and go out way too hard. Before you know it you are in a massive hole that you won't come back from. I should know, I've done it.

So pacing is critical and the training is as much about ingraining that as it is about getting stronger. i.e. knowing and testing your limits ahead of time.

The plan does assume you have some base of fitness and that you've been doing at least consistent training 3-4 times a week for several months.
I wouldn't suggest taking on this plan if you've not done any general training up to this point. In fact if this is you then move your entry to next year and start the training for that now.

I've also assumed you have a good idea of what your current threshold power is from a recent FTP test, if not then i suggest you do one before you start. A ramp test is probably the most effective way to set this. If you're not sure drop me a message.

All of the sessions are power based other than the long ride scheduled for Sunday's which is heart rate based. Given the duration i think heart rate is better metric to follow along with your own RPE (rate of perceived exertion). I provide more detail around this in plan.

If you follow this plan 90-95% to the T then you will do well.
Good luck