Two things from me:
1. I see the forum as a way of engaging with the wider membership. I will take the concerns raised, and suggestions aired to the NEC for discussion.
2. John - Great to see that you're thinking of organising an event. I have added a link to the helpful guide to walk organisers. It is dated but is still sound.
https://www.ldwa.org.uk/library/guidelines/guidelines.php
To access the LDWA public liability insurance, you'll need to get your local group to put the event on and you would be the organiser. I see that you're a member of the London group, and some discussions of your event on the social walks would be the best way of getting interest from your fellow group members and enthusing them to support your quest to put on a 50 mile event..
There is no minimum height of ascent and unless you are arranging a kanter, then you'll need to ensure that you have regularly placed checkpoints.
At the risk of hurting your feelings on your route choice, you might wish to be a little more creative, as I can't see many people entering an event that goes up a canal and returns by the same way. Neither does a route such as you have described test the navigational abilities of an entrant at night time. But, a canal combined with other countryside might be attractive to other LDWA members. I quite like a canal walk and have entered the Birmingham Canal Canter this year to explore the city by our oldest form of motorways!
If you get your event off the ground, I look forward to seeing it make the LDWA calendar and join other events that would be a suitable qualifying walk for the annual 100 mile event.
Regards,
David Morgan
100 Coordinator