Congrats on your new cub.
Welcome to the pain train.
Note: I am not a coach nor expert in anything here. Just an enthusiastic training geek. I will write in an affirmative tone, but this is just my understanding and opinion. Note that your goals and these comments reflect my actual training as well.
So, for reference, I’m 6’ 3”, 100kg (lean). My FTP tests at 290 and I have it set to 285 in trainer road. (It is 306 in Zwift). I have been on TR for 5 seasons. My FTP started at 190, with big gains in year one and two, and now very hard earned gains. I generally train at Low Volume, but I also Fatbike and ride Zwift races 2 extra days per week so my volume is more like Mid.
You have a lot of muscle mass on your frame, as do I. I will suggest that you probably have a lot of endurance strength in your legs (because we have a lot of slow-twitch fiber), limited lung capacity (oxygen delivery to your muscles), and limited high-intensity performance. (Without oxygen delivered to your muscles, they can’t perform super hard efforts, which are above your FTP value).
I think you will likely find the easy parts of the workout, very easy, and the hard parts, good hard work. General training theory likely says that you need to build your oxygen delivery system up, to better fuel your hard effort work. This means more Base Build work. (Like another 8 weeks would be good). No point in building 600 watt sprint legs if you only have 200 watt lungs and heart. This is achieved by training the heart to fill and pump more blood with each beat, which requires a good amount of low intensity training at Base Build levels.
For a long race like you have identified as your goal (which I think is too much in your first season back btw, is there a 50km category?). you will need to build greater endurance capabilities; you want to have good steady, strong output (sub FTP 80-90%, for long durations with limited periods for recovery). So I would say an endurance oriented strength build would be your next target plan. (Maybe sustained build?). As an additional element here, I will suggest you need to build your normal cadence up toward 95rpm. (This took me 2 seasons of TR)
Don’t worry about testing your FTP again. Just raise it +10 and do some more workouts. Easy intervals and workouts are going to be easy still. If the hard stuff feels a bit harder, but YOU COMPLETE ALL THE INTERVALS, keep going. Up the FTP until it feels harder, only a few watts at a time. IMO, failing to compete workouts is bad. You WANT to complete all the intervals and finish the workouts. Adjust FTP accordingly. (This is one reason my FTP is set slightly low). Test again at the next training plan test.
Make sure to maintain your CONSISTENCY of training. Low volume is fine, but make sure to stick to it. If you only have 30 minutes to train one day, swap the workout to a short variation and do it! Consistency is key to muscle adaptation. The periodization of these plans is part of the magic. Rest weeks are there for a reason. (Trust the Chad). During recovery weeks where things feel too easy (possibly because your massive build gives you disproportionate endurance capabilities), you can swap the workout for a LONGER version of the same. Training stress is (generally) measured by intensity & duration. So increase the duration to get more workout. (Increasing the intensity might pull you out of the base build zone where you are building oxygen capacity). I would suggest you continue to base build, especially if you’re set on a 100km, 12hr race.
Nutrition: No discussion on improved training is complete without talking about nutrition. The guide I use is listed below. It is written for a different set of training plans but contains all the information you need to properly fuel your body for intense exercise. Don’t try to change everything at once and don’t overthink things. You need carbs and protein. Try making a few sensible changes to improve your overall diet. This will pay big dividends. Pay attention to the section about losing weight; losing weight while effectively fueling the beast is a paradox but can be done by using wholistic view of inputs and outputs. Depending on your physiology, you will be able to drop a few kilos helping your power to weight ratio. (Watts/kg).
Good work getting started and making changes.
Happy to share my thoughts anytime.
Enjoy the ride.