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With rides like PBP and LEL the biggest problem would be no access to the controls. Whilst it's perfectly possible to source food and sleeping arrangements elsewhere, it'd be a huge pain in the arse.
Also, the motivation for "just doing it" is going to be less when you're not being tracked and your name won't be on a list of people who completed it.
As an example: running the London Marathon route without entering, or on a completely different day, doesn't get you on the list of official finishers. You can still post it on Strava though and you've still run a marathon.
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Also, the motivation for "just doing it" is going to be less
With an ultra, this would be a massive issue. It's such a mental battle to get through these things that you often ask yourself 'why am I doing this?'
If you can't give yourself a good answer that makes you feel really good about yourself and what you are doing, then you are fucked and you are out of the race because you won't want to carry on!
With these events, p-b-p and events like dragon ride, if the route is freely available, or/ and is signed what is to stop people just following along without entering?