It took me a while to learn how to eat on a longer ride when I started randonneuring. I did a lot of longer rides as a teenager, but I also dragged myself home in a sorry state a number of times. On my first 200k, I pretty much survived on gatoraide, but it turns out too much of that upsets my stomach, so the last 30 miles was pretty dire. Fortunately it was relatively flat. This is an ongoing experiment of one for many of us. I'm glad I did a number of 200km rides before I did anything longer.