Training plans are all well and good, but the first goal is to be able to ride 9 hrs.
There is no substitute for doing long rides to prepare to do a long ride. In my experience,
for just developing pure endurance you get 80-90% of your training effect from a long ride. The rest of your training should be focused around reducing the recovery time to enable you to do your next long ride. There are also lot's of logistics around eating and drinking for 9hrs on the bike that you'll have to figure out as well and there is simply no way to know what will work for you without getting some 5-6 hr rides as practice.
Given that you'll be inside the next 3 months, you should probably focus on consistency with some intensity work. Do as much cross training as you can. It's very hard mentally to do significant endurance training on trainer. If you can find another activity like XC skiing or a similar winter endurance sport, that would be beneficial. The most important part over the winter is to stay consistant and to not mentally burn out. You want to be raring to go when the weather gets good, not disgusted with riding due to burning out on the trainer. Going to the gym to work on flexibility and core strength is also a good winter activity. 9hr rides will make muscles painful that you normally don't notice riding.
Once the weather gets nice start working towards a regular extra long ride at least every 2 weeks, every week would be good if you recovering in that time. Start at 3 hrs and add a half hour until you start to reach a point where you can't recover in time for the next ride. Stay there for a while until you're ready to make the next increment. You'll know you're ready to move to the next level when you're excited to do a long ride, if it starts to feel like a job or an obligation, that's the time to step back.
Endurancing training is as much about managing recovery as it is actually training. People with the mindset to enjoy endurance sports rarely have trouble with training, but they almost all struggle with overtraining. Learning when to push through and when to back off takes a very long time. As a relative beginner, I'd urge you to err on the side of caution. Taking a rest week every now and then should be part of your plan. Taking a week off will not significantly affect your fitness at all and might provide suprising benefits. Overtraining that extra week can destroy months of training with injury and illness.
300 km in 9 hrs is a very ambitious goal. If you want to be confident I would want to be
doing training rides of at least 6 hrs, 7 or 8 hrs would be better.
You'll also need to taper at some point before the event. Plan on at least 2-3 weeks of reduced effort leading up to the race.