Ingo just suggested that my 4:04/km might have been an outlier. Which is a fair question. But it looks more like a trend to me. As I said, the tests both before and after were also supported by the general trends on other runs. And I worked my arse off to make that trend happen, so it is all very real to me. What I will admit though, is that it is just a standard test for some kind of quantitative tracking of fitness over time. What I don't have is a reliable way of translating that test result into a target race pace for any particular distance. I am not for a moment suggesting that it meant I should have been able to run a marathon at 4:04/km. In fact I am now pretty certain that I over-reached when I went for sub 4:10/km pace. But I am OK with that because I decided to risk the sub-3 result by taking a shot at a PB, and to get a PB I had to run sub 4:10 pace. And for one reason and another it just wasn't to be.
All this does raise the question, of course, as to what is a good predictor of your current capability? Especially for the marathon where going out hard and hanging on simply doesn't work. Arobically, I was in as good a shape as I have been in, but it didn't translate into overall endurance because the build-up had been too short and there were not enough longer endurance/stamina runs, and perhaps not enough taper. So given that tests like this give a good indication of aerobic fitness, I wonder what you can do (apart from running the marathon) to adjust your upper aerobic pace to account for lack of endurance?
This is especially important for some of those fast-twitch people who blat around for 5k or 10k, but struggle to put together a marathon worthy of their potential, usually because they short-change themselves on the necessary endurance work. OK, so how do they adjust their 17:30 5k capability for the fact that they have only done two runs over 30 km? This is where McMillan calculators and the like break down and/or get abused leading to those horrible blow-ups in races that I have seen happen so many times.
What I am suggesting here is that a 3:10 marathon with even splits is probably a better marathon than a 3:10 with a 2:50-pace first-half and a 3:30-pace, horribly painful, part-walked second half (Dan, Martin, care to comment?). So how does Mr or Ms 17:30 5k runner convince himself to go out at 4:20/km instead of 4:02/km?? For me, the situation was not quite that dramatic, but the same principle definitely applied.
I suppose the answers are out there somewhere in coaching-boffin land!