I knew someone like that. He was coming out with me on 2 hour rides a couple of evenings a week after work. When he got fit enough to do more, I asked if he fancied doing an all-day ride one weekend. He looked at me as if I were mad and told me that 2-3 hours would be his mental limit. He was sure that physically he could ride all day, but he hated the idea of doing it and said that he would be bored stiff ...
I don't think that I have ever been bored on a ride. There have been times where I didn't feel very energetic and thought that I might have been better off resting, or maybe the weather turned bad on me. A couple of times I really suffered, especially when I was becoming seriously ill but didn't know it at the time. Bored though - no, not even on the (few) 14 hour rides that I have done!
Strangely I don't get bored on my long mountain hikes. I do quite a few long days in the local mountains of 8-12 hours and love (almost!) every minute of it. My mountain buddy and I intersperse the hiking with bouts of scrambling whenever we can - which we both love. Later in the year we are doing the Three Counties in one day ie Coniston Old Man, Scafell Pike & Hellvelyn which is 35 miles and I know for sure we won't be bored - knackered but def' not bored.
I'm ok with cycling but it is an adjunct to my main passion (hiking) but it is a long way from exciting for me. It does give me some pleasure, quite a lot at times, but at some level sit and pedal, pedal, pedal does not float my boat.
I have no doubt that with a bit of training I could cycle a long way although the prospect does not overly appeal. 30-35 miles is about where I get really fed up although i am nowhere near knackered at that point.
Mind you, I'm well aware the same could be said by some of long distance mountain hiking....one foot in front of the other = boring to some!
Each to their own as ever.