The first 200k I did, I went out with Chris' (Ram) group, but thankfully didn't make the cut at the first control, so then soloed most the rest of the way, on a 35° and rainy PNW spring day. The last 50k got kinda hard. My best riding buddy dropped off of the #2 lead group and waited at the last control, because he knew I was only like 15 minutes behind. Then he babysat me in. When I got in, I was telling one of the Real Rando riders that there was no way I could ride a 300. His reply was that I had made it in under 8 hours and I absolutely had another 100k in me. I thought about that for a long time, and finally realized he was right. I soloed the 300 in decent time. It was hard and the last 20k hurt pretty good. That's how it goes. If you like that, it's all good.
There are really two mental aspects. The first is the preparation. It helps to be analytical. You have to figure out what will increase your speed and endurance, and then carry out that plan. It helps if you were right. Then there's the suffering aspect during the ride. That's just a type of spiritual purification, not that different from the rituals in other tribes. Remember: they don't call it the Dark Side for nothing.
The program the OP envisions is exactly correct. Get after it right now.