I tried potatoes for some of my long rides too- I found that they are just not calorie dense enough- ie not enough calories per volume.
I need about 250 cal per hour I'm riding and I was suprised at how few calories potatoes had!( 78grams=65 calories) I just couldn't physically carry enough- it is too bad because they sat well in my belly
I'll bet that you just didn't have enough overall caories and that is why you crashed part way thru- it is a good thing that they have a high GI since you want to access the calories right away- but you need enough of them.
hope that helps
The cure for anything is salt water;
sweat, tears or the sea
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