Thanks for your input!
For what it's worth, I've worked out why this is much harder and slower work than 'Broken Bow'. For my first story, I had a pretty good idea how it would all progress and it was just a question of writing it all out. As I did so I would sometimes get ideas for extra bits, or find some elements needed explaining (such as Mayweather and Locke's attitudes). This slowed the writing process down, but (hopefully) meant the final piece was more polished.
With this story though I had a much less detailed concept of how the story would progress. I know what is happening and when, but I'm not so sure as to how to get there. As such I'm relying more upon moments of inspiration to fill in the gaps.
For example, this chapter was inspired by two programmes I saw recently. One a documentary about the USMC and the selection process for Recon. The other an episode of 'Dirty Sanchez' (think a British 'Jackass' only much, much worse) where the lads had to face a Royal Marine training course.
Together those got me thinking of some sort of endurance run by the Pathfinders, and I went from there. Fortunately I was able to tie it in to the main story, so it's not (entirely) filler.