Sorry to hear about the afib, I too have had some very serious heart problems and got back to racing. My SCA (VF) occurred at the end of a 4k (sic) race. I won't try to persuade you to change your mind about the advice but it would be interesting the next time you see this guy to ask him to explain why he thinks the duration of the event is more important than the HR level?
As for the 'the whole public adoration thing that only marathons seem to get' are you serious? You're talking about the perceptions of non-runners. Have you seen the WAVA scores for your times across different distances (easy to calculate online if you haven't) and can we ask which distance you get your best WAVA score at? If you say you like HMs then do HMs! But that will be a higher race HR than your marathon...
The training for the two events needn't be that different, depends what sort of quality work you have been doing. You can keep all the mileage and long runs going and drop the length of intervals/tempo sessions to build a bit more speed. Why do you say you don't like the look of the plans you saw? Not sure you're supposed to!
My best recent 10k is 41.26 but my training looks nothing like that. I'd say you need more focus on steady runs and slow down, and don't try to do flat out runs like that. Also for races don't go off at an unsustainable pace, if your km reps are only 3.45 no point in dashing off at 3.38 even in a 5k. Reps can be done at different intensities but for me, km reps are at or above 5k race pace.
RY's sponsor SKINS have accepted the conclusions of the report and terminated their relationship:
"The conclusion reached by Roger and Ross is one I accept.
However, it’s also the case that Rob continues to vigorously defend himself. He maintained to Roger and Ross that he ran every step of the way, but he made mistakes with the collection and handling of data.
Nonetheless, we have terminated our relationship with Rob Young.