If you can ride 300km you can physically finish a 600km, the only things that will stop you are the head and the gut. Above helps me with both.
I can do a 200 without eating. It is neither pleasant, nor an experience I would recommend. (Nor was it intentional). But that is simply not sustainable when you start talking about real distance. Something I've really struggled to learn is to eat. When I do long rides by digestive system shuts down and just doesn't want anything beyond water. On the TCR I got to the end of the start parcours and found a gas station, i sat staring at a muffin for over 45 mins trying to persuade my stomach to accept the food. I was in a bad way with the heat by that stage. I thought I'd drunk enough, but in hindsight I'm not so sure.
I have found that generally M&M's and coke can always be poured in. The former are less useful when it's 43°C...
i have my garmin reminders set up prompting to drink every 20min and eat every hour.
Now that is a feature I'd love my wahoo to have...
Don't dash off too fast Rainbow Dash! I always have a better ride if I've ridden in slow-ish to the start, ECE'd or not, as a gentle warm-up. Think tortoise, not hare; start at the back of the field, and don't get sucked into anything competitive; you might pass a lot of the speedy riders later on! [Yes, I know for some it's always a race!] But look at the way the pro teams shelter the GC riders and sprinters, and burn out their domestiques - don't emulate the latter!
It has become accepted within RNL, that I am Lantern Rouge. Everyone else is just so much faster than me. But at the start if I can get into the middle of a group, I can usually get a few km in at a 50%+ faster speed than I do on my own. I know I will fall out the back at some point, usually on a hard 90° turn where I just don't have the sprint necessary to get back on. Tho my Amsterdam attitude to junctions, and traffic lights has lead to me jumping 16 other riders in a single junction... (oops). Riding in a group, esp in strong winds, can be really effective at energy savings, if the group is going at a speed that doesn't require you to burn too many matches.
I find on longer rides it also helps to have a good awareness of what the weather forecast is going to be. This helps a lot with the gauging when to use your limited reserves. If you know it's going to be 200k into the wind, then turn for 200k of tail wind, you know that you can put a lot into that first 200k, eat a good meal, then enjoy the wind home. But if it's going to be the other way round, you know not to give it too much on the first bit, so you have the reserves necessary for the headwind slog.
Some people like to recommend breaking up a ride in their head into multiple short sections "I just need to do 60km to CP1" "Now I need to do 70k to cp2" etc... But I find that the last 20km of any ride is a slog, and doing this just means I have 4 last 20km's on the ride, rather than 1...
TLDR: Drink more water, don't forget to eat.
J