This leads on to something that buzzes round my head when I'm riding.
You don't need to do a named challenge like LEJOG or Dungeness-Durness or Calais-Brindisi to do a challenge ride. Nor do you need to enter an audax to ride 200k or 300k or whatever. You could just make something up. Those labels are just stamp-collecting. But you may need the high level motivation provided by the idea of a finisher's medal, or hoarding audax points or having recognisable bragging rights, or a little star in your CC sig line (my particular weakness).
To comfortably ride a long distance (and "long" has different meanings to different people) you sometimes need to forget the big picture. If your legs hurt, then pondering the fact that you are only 132km into a 200km ride with a huge hill at the end is likely to lead to despondency. On the other hand, pondering the fact that your legs are not actually dropping off, and are still working, and can continue working for at least one more minute, will help. What happens after that minute can look after itself.
Once you're on the bike your motivation needs only to keep you going, not get you to the end. If you keep going, you'll get to the end eventually. That instantaneous motivation can come from anywhere, from the sheer pleasure of riding your bike. So at a small scale, the big targets are irrelevant and can even be a hindrance. You're tempted to say "Hell fire, that's too far for little me, I won't bother at all".
But for many of us it's the stamp-collecting that gets us out there in the first place, even if it can also be a daunting and de-motivating factor. We need the structure and we like to collect stamps as well as ride our bikes. The trick is balancing the big and the small.
Ask: Do I want to do this, or do I want to have done this? If you can answer yes to both then you're half way there.
Sorry if that was a bit airy-fairy. As you were.