Impact on the feet can be stressful for the underside of the feet. Yes, cycling is not as high of an impact as running, but it still has somewhat of an impact. I have stress fractures that continually reappear, unless I rest my feet. So I make sure I massage them daily and do stretching specifically for the feet, toes, calves and shins. Try the book called "Stretching", by Bob Anderson. There's a section in there dealing with feet stretching. Always work on massaging your feet (I'm massaging them now as I write!) and stretching, and don't be afraid to put some ice packs on them and elevate your feet as much as possible.