In reply to bouldery bits:
Hi BB. I did the Glen Coe Skyline last year, which was pretty different to anything I had done before (longer and much greater height gain). Similar to what you describe, I prepared by adapting a book aimed at alpinists, of which I am not one: Training for the New Alpinism. I found this pretty easy to adapt for my needs for the GCSL, which I should think is probably not entirely dissimilar to mountain marathons and adventure racing.
Fire away if you have any questions about it.