hello
although it is your feet that are cramping.. what might the cause of the cramping in part.. is that your legs need to be properly stretched out........ especially AFTER a big ride... as you can well imagine how tight one's calves quads and hamstrings can become as well as tendons which run down the leg...
I highly recommend doing a FULL body and thorough leg stretch. My feet used to cramp up all the time too UNTIL I began to do thorough stretching.... which included not just my feet but all of my body.. including ones hips... when you hips are tight, they cause the entire leg and foot to become misaligned.. and as one ages, the less you do about this, the more prominent it gets.. ever notice people whom when they walk , they usually walk with their feet pointed either to the right or left , generally it's not straight forward which is how your feet are meant to walk.. take notice today when walking, do you favor a specific foot or even both .... the more your hips turn outward ,(you see this a lot in pregnant women) the more the feet point to right and left. try and begin to notice and correct it if you can because this will definitely help your posture AND the cramping...
imagine if walking or sitting or even riding a bike but doing so out of alignment, well of course one's body begins to contort and cramp...
by becoming aware, you might begin to notice a big difference.. also, yoga really can help with the cramping problem..
best of luck to you