Cate - thats a good idea re.preparing food in advance. One of the difficulties with a multi-day like this, where you will potentially be out for hours and hours (unless you are one of the few speedy racing snakes at the front), is getting enough 'real' food down your neck. I was getting up early, having a cooked breakfast, then 30mins before start having more toast, etc, OH would have dinner cooked already by the time I finished so I would trough that almost immediately I got in, then snack all night.
Day 3 will truly test your desire to finish. I wouldnt worry too much about accumulating big mileages at this point, it is probably more important to be fit and well rested before the event, although if you know you can recover quickly there would be no harm doing say 15, 21, 10. Having the desire to get out of bed and run again the next day is the key. It is useful if you can go out with fully loaded pack and walk, or yomp, up some big hills then practice descending. I walked quite a few of the descents on day 3, mostly large sets of steps with big drops, which my knackered knee found hard to cope with.