Appreciate the responses above - it's actually very helpful to receive differences of opinion/approach so keep on disagreeing!
I took a lead from Smoove yesterday (as it ticked a number of boxes for me in my training progression) - 2 up, 4x1 at Tempo (5.30-5.35) off 90s, 8 'easy', then pick-up again for 3 (avg. 5.50), 2 down for 20 total. Some observations: it felt good to be covering that distance as a structured session, not just a run out; getting some quicker miles in early changed the feel of the run, having to run on legs that had already been 'used'; having said that, the middle 8 'easy' felt solid - the first mile after the last Tempo effort was 6.45 and then progressed down naturally to 6.10 (avg 6.20 for the 8); of course, that was probably a bit too much and so the final pick-up was tough and wasn't able to get near to Tempo again; finally, without really thinking about it I covered 20 miles in ~2.08 including up/down miles.
Marathon stuff is such a steep learning curve - it's great, but I'm beginning to see why you need several cycles to work things out!