I can't agree that that is ALWAYS the problem, esp. for a first-time LONG rider.

My first 300k brevet, at about 140-miles, I hated the bike, my hands hurt, me feet hurt, my butt hurt, I could not understand WHY anyone had ever ridden a bike, I knew that IF I finished the ride, I would certainly never get on a bicycle again for the rest of my life!

Within 10-miles, I found myself thinking, "I can do this!" I was not referring to the 300k -- I was referring to the 400 and the 600 brevets.

What had happened to change my outlook? I hadn't eaten anything; maybe I had sipped some water.

Here is what was different: there was a light rain falling, a warm, Spring rain, yet it was cool enough to cool my legs; darkness had completely enveloped me, and I was concentrating on seeing through the dark and the rain. Maybe I had relaxed my body and arms/hands and legs/feet a bit. In other words, I found a comfort zone.

After the ride, I did put some additional tape, with more cushion, on the handlebars; a few months later, I replaced my falling apart shoes. But mostly, I think I learned a lot about relaxing while on the bike. And, I learned some of "what to expect."
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Btw, I have nearly run out of energy on a rando ride. I didn't hate the bike; I just needed to finish the last few miles and get out of the cold and eat (in the one case), and ride a few miles to get somewhere more amenable to a serious stop-and-eat.

I have also "bonked" on a ride (American def'n of bonked, not English def'n). I bonked so badly that my buddy decided to go get my car (which was only 3 miles away); but before he left he made me promise him that I was not going to die. I thought he was joking -- turned out that he was not joking.
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Hating the bike, etc. may mean nothing more than you are sore.