Here's an idea. Instead of trying to follow a sub-3:15 plan, but not being able to do the paces, but then compromising by making up random paces you're going to aim for instead... why not actually work out what the equivalent paces are for your ability level, using the McMillan calculator and do the same plan, but with all those paces subbed in for the plan's pace guidelines?
Oh, hang on, I suggested that several pages ago. Never mind.
I am not an expert, but by amazing coincidence I ran my first two halves in similar times to Runneralex and my most recent one in JB's target time, so can claim some experience, if not knowledge. I prefer Alex's programme myself; JB's seems to have too much quality and especially the three days in a row (even though the Friday session is not very fast).
Also, JB, what you call your 'tempo' pace looks very slow (somewhere nebulously in between marathon pace and easy pace) for your 5k time and HM time goal. Which is probably just as well given that you're hammering the hard sessions and hardly doing any easy running.
My best improvements came from higher mileage and more of it easy.
Having spent 7 hours in a removals van yesterday I can confirm that it is indeed a long way from Carlisle to my new house, though approx. 10 miles nearer than it would be to St Andrews. I had a wee behind some bushes in a layby in Cumbria, Skinny, and thought of you.