Naisme I had that same thing happen to me last week except I was riding in and just felt bombed. I had no strength left. I attributed it to riding too soon after doing my double century but I came down with what it sounds like you have. Took me four days to shake it and one and a half of those days I ran a mild fever as well. I found 15 miles wasn't too bad but 25 about killed me.
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Sunrise saturday,
I was biking the backroads,
lost in the moment.