Commiserations on the blisters and the disappointment.
I've never been able to fathom why, once in a while, I get blisters under the heel.
Same boots, same footbeds, same type of sock - usually OK, no reason to expect a problem, then out of nowhere I know there's a blister forming.
Your point about the pace of walking sounds like a good one. My problems have always arisen on warm, dry days when I've been doing higher mileages than normal, but probably aiming to finish at the same time. Thanks for that enlightenment: the blindingly obvious isn't obvious until someone points it out!
As for the flat footbeds, I'll take a look at the article and might be tempted to try them, but I start from a sceptical position.
_________________________
Pete