Everitt- I understand what you are saying and I do agree, I think for my own piece of mind I would do some practice just so I know I am comfortable kayaking and also to help with the upper body strength. I did 2 miles on the River Thames on Saturday morning and my arms did ache a little towards the end. I think the kayak section of this C2C is 4 miles or more.
Steven - I have done 1 off -road duathlon and do hope to do some more in prep. And I would certainly be interested in seeing your training plan as an Idea for my training.