@frank9755 was in the Trancontinental Race this year and wasn't using SPDs. Have a look at what he wrote in post #82 of the Transcontinental Race Heads Up thread.
I'll have to read this thread through as I expect I'll learn something useful from it.
But
@slowmotion is correct, I've abandoned cleats for long distance riding. For TCR I used some Innov-8 trail running shoes. They weren't perfect but I tried 7 or 8 different pairs of shoes and they were the best.
I won't go through them all but the things I look for are:
- wedge-type sole as I pedal midfoot - I don't want a pronounced instep in the wrong place
- zero heel drop. There is a search engine for runners which sorts trainers by amount of heel drop.
- reasonably stiff sole. But this is not that essential as cycling insoles - I use Specialized - make them stiff enough so that I don't feel the pedal too much. The other thing that I have done is just cut out a piece of flat plastic to go under my insole. That makes a noticeable difference.
I've tried a few pairs of Five Tens or similar shoes from Northwave, etc. They are good on dry days in winter, but they are all way overbuilt for on road cycling, so they are too hot for me in summer. Also they are horrendous when they get wet. Soak up loads of water and take ages to dry. Also I find Five Tens a little bit too grippy - can be hard to make fine adjustments to my foot position.
What I would really like to try is an SPD shoe with a flat / wedge sole, where you have the option to cover up the cleat hole on the base, and where there isn't a big raised plate pressing into my foot on the inside of the sole. I tried a few likely looking options but couldn't find on that met those criteria in time for TCR. It may be that an SPD shoe would be too stiff so wouldn't be grippy enough, but would be worth a try as I don't like to be able to feel the front and back of the pedal too much.
The other thing that I am going to try is carbon insoles. I have a pair but not got round to them yet. They might be risky for long distance as ultra stiff.