As I came in US 30 from Longview on my first successful "one day" STP, I saw, littered along the banks of the highway, the sad "road kill" of folk who were unable to proceed.
They looked exhausted. Bikes tossed on the ground with little thought. Blank, dazed looks on their faces as they stared off into the distance, usually down the road towards the finish 35 miles away. Sitting passively: their arms around their knees or supporting them as they leaned back or lay on the ground. A group of 5: one lying on the ground as another was pushing and pulling on his legs as if to relieve a cramp and the other 3 standing around in an arc watching.
That day, I wasn't one of them: but I have been! I recognized the cramping from lack of hydration or electrolytes. I saw the exhaustion/bonking from insufficient food. I heard about others with the bloating and stomach upset due to not processing food.
Yes, there are many ways to fail at riding a bike for 200 miles in one day: I think I've done most of them. As they say: