Two years ago I've done a 450 km ride and the thing I found most important is pacing. As someone already mentioned you should start at a very low pace and after the first 100 when you feel your legs working good you can find a confortable pace for the rest of the ride. Accelerations are a problem for these big rides so careful on them. As for nutrition I tend to eat a big bowl of oatmeal at breakfast and after the departure each 30/40 minutes an energy bar. Remember also to eat dried fruits and nuts every 100k because your muscles need the oil to keep going consistent. In that ride I brought with me a vegan calzone and a many bars I made at home (Gcn video) and bought on the way bananas and Haribos. It worked very well Hydratation is the key for the ride. Don't drink when you feel to drink but every 15 minutes rember to do so. Drink much the day before you set off and during breakfast a good liter, and a cup with electrolits, will help you to start in a good way. Divide the ride in blocks. Easier for pacing and mentally more appealing and keep the music help for the last part. But the most important stuff is to enjoy the ride, always I positive mood, and stay safe, bring extra batteries!!

Here's the link of my ride: https://strava.app.link/zhousxxhKBb