Adam, I used the same plan for my first marathon. I basically ran my mid-week runs at the same pace as my long runs, i.e. pretty easy. What I would do on the odd occasion, and not more than once a week, is add in a proper tempo run, but this would be a very distinct pace from the easy runs. So at the moment, maybe you're doing too many miles in "no man's land"; a bit too fast for an easy run, not allowing you to recover adequately for the next long or hard session, but not quick enough to have a useful training effect e.g. improving your lactate threshold as you would for a tempo run, or even practising marathon race pace (which I know HH includes on the odd Saturday run.)
So, slow down the majority of your mid-week runs (and take the odd rest day if you feel you need to) and this might help matters. Something like 9:30 for easy runs, 7:30 for tempo runs, and if you do end up running at 8:30/m, this should probably only be towards the end of easy runs if you're feeling particularly good, or scheduled for a specific "race pace" session.