Jeremy, I don't know of any challenge walk which is held over three days, but there is at least one over two - the Saltmarsh 75, around the Essex coast each October. Pub dinner/breakfast and camping are available at the overnight stop.

hi it is so easy to make up ones own route,just look at the maps ,apair of divders measure the route,look up the campsites or B/Bs and create your own long distance or pick a well know path. ie The monarchs way etc,artie

Jeremy, if you decide not to do the 100 next year, you could offer to help.

I have done 10 100s, and have to make the decision whether to do any more!
I expect that it is very pretty in S E England around Dover and surrounding area.
However, might instead go for a 100 mile trek over say 3 days at 30/35 miles a day, so just walking in the day time, in a more hilly environment!
Are there any events of this nature that LDWA put on, or would it need to be a private arrangement.
Logistically, quite difficult, as one has to decide where to spend a couple of nights.
Alternatively, an event such as the 3 rings of Shap with some changes could suffice!
Food for thought!