Wild camping may not be too much of an issue, provided you ask the land-owner. We had to wild-camp twice on Wild Edrics Way which shares part of the ODP route around Clun/Craven Arms. Just asked the land owner about camping of one of his recently mown hay-fields each time. With no crop on and nothing about to be planted it was easy both times.
Pack-weight may be a problem for you, if you're also carrying sketching materials. - My other half is a semi-professional landscape artist and her sketching kit weighs at least 2 kg, before the inclusion of additional water for paint mixing.
Your basic kit (asuming each person carries all their own kit - i.e not sharing tent/cooker etc) will probably end up weighing in at around 15kg. Plus food, water and artists materials - could get to 18 kg plus!
I'd try it out on a shorter (2 night?) trip first - so that you can work out what to ditch for the real event. Nearly everyone I've ever back-packed with has sent some kit home after 2 - 3 days, because they've realised they're carrying too much.
A breakdown might be:-
Tent - 2kg
T'rest - 1.5kg
Trainers - 1kg
S/Bag - 1kg
Cooker/Pots - 1kg
W'proofs- 1.5kg
Fleece - 0.5kg
Spare Clothes - no more than 2kg
Rucsack itself - 2kg
Plus wask-kit, first aid kit, maps/guide-books torch, spare batteries etc. etc. it soon adds up - then add your lunch and 2 litres of water!
I find that upto 17 kg is manageable - you get used to it after 48 hours - more than that and its a permanent struggle!
LWM - will no doubt be able to cut those weights back a lot further - I'd be just as intrigued, as you are, to find out how!
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