The more details the better. Contrary to the popular cliche, a picture is NOT necessarily worth 1000 words, though pictures do help. Pictures trump words where the writer is not all that good at it. Keep detailed field notes. After the tour, when you have time, refine it. Many revisions and knowledge and practice in good writing may be required. It could be time consuming. If you are going to take the trip of a lifetime, make many copies of your well written journal and pass them along to future generations.
If you don't already know how, learn description. Keep a lightweight, digital recorder for capturing details as you go along. Try to remember details of conversations. I have extremely detailed journals and some less detailed. For cyclists, vivid descriptions of road conditions and traffic would be of some interest. Good writing can make most anything interesting or most anyone. If you are all that serious about it, you might want to seek out a night course in creative writing. Actually, good writing can be hard work, quite time consuming, and require a lot of serious thinking, and also the reading of other good writing in your subject. For reading, you might want to start with "The Great Ride," "Miles From Nowhere," and a book written by an Englishman many years ago which has been described as the greatest book about round-the-world cycling ever written, but I don't remember its title.
The journals on CGOAB are fine as those sorts of journals go, but if you really want to produce your magnum opus, you would not use them as models. They are somewhat interesting and engaging and imaginative. I have read many CGOAB journals, and while they are OK, they are not what I am trying to tell you to do, but several grades below. It's up to you what you do and how you go about it. You may eventually decide that the time required for putting together a really well-written, detailed journal is not worth it. I have written many journals and many hundreds of thousands of words and studied good writing and I have taught it. Revision is a key to good writing. Books are not written. They are re-written.