Good for you for giving it a good shot. And even better for you, for doing a good analysis of what didn’t work and what to do next time. As a couple of others have mentioned, doing an organized century ride is much more enjoyable due to all the distractions and food/water stops, unless you don’t like people. Another thing to start training is more hill climbing. If you get good at climbing in advance, then on the big day, it will be much more easy. There is a saying I learned many years ago, “To win in the hills, you must train in the hills”. But good on you for looking for a flatter route in the future - and good climbing legs will help you there too.

Best of luck and keep us posted. Most people don’t have the courage to detail their failings, so hats off to you a maybe another budding centurion can learn from you.