My longest one before this point that I recall was around 10-11 miles, and I can pretty easily handle 8-9.
However, I should add that those have been without extra supplies. For the 25-mile one I wound up consuming 2 bottles of water and 1 protein bar during it. (I'd packed two bars but the other wound up unused.)
Sounds like a simple matter of conditioning. You simply weren't prepped physically for a ride of that length. Because you'd never ridden that far before, let alone ridden that far on a regular basis. So yes, sore feet.