RossRobObey wrote:I will do a short first day, but how will I know how many miles I should do before the next stop?
By the time your start date comes nearer, you will have a fair idea of what is likely to be a reasonable mileage for one day and can plan accordingly. As the journey goes on, your fitness will improve giving you the opportunity to increase your daily mileage. I think it unlikely there will be a YH conveniently placed to suit you every evening, you may have to consider B&B.
Pre booking can adds to the stress if you have a breakdown or if your daily mileage exceeds your estimates, and you are tied to prebooked accommodation.
Tourist info offices can book accommodation ahead for later in the same day for a small fee, but I would suggest not booking anymore than 24 hours ahead. By midday you will have a good idea of how far you will get that day, that is early enough to be thinking of where to stay overnight.