adders, my experience from the tail end of the mid-pack blessed with general purpose genetics and needing 2 failures before nailing it may be ones you wish to (and possible should) ignore, but you are getting them anyway :-)

Get out en route with someone who has done it or knows the route well and do 2 legs. I had a internet arranged blind date with Penrith Stu in Feb 2010, we did legs 1 and 2 on a snowy day and it really opened my eyes to what is needed. As the Cumbrian put it, I was "on my arse" by the end of leg 2. Once you have some idea of the scale relative to your ability, you will work out for yourself what you need to do. It was a rare week where I did more than 40 miles. 10k of climb was common, but if I raised the miles I got injured.

Train to eat. Learning what I can eat was more of a factor of success for me on attempt 3, than any difference in fitness.

I have paced a few rounds where the ability to descend was a real limiting factor towards the end. Don't neglect the descent aspect and getting comfortable with relaxed descents
and getting the muscles used for heading down strong.

There is lots more to learn, but the fun is finding it out for yourself. Enjoy the journey.