Well yes, walking too far with fairy soft feet is one cause - however, I think I have found another.
I was wondering why my hands were getting swollen after 20 miles or so. It was quite pronounced and something I have never experienced before. My training partner Bryan was also experiencing the same thing.
After much research I have discovered it's caused by an electrolyte imbalance. I can now tell you with some authority that electrolytes are chemicals - Sodium, Potassium, Calcium, Magnesium, Chloride, Phosphate and Bicarbonate. Your body gets them from the food you eat naturally. When you exercise heavily, you lose these electrolytes in your sweat, in particular, sodium and potassium. They have to be replaced to keep the electrolyte concentrations of your in you body balanced.
One sign of an imbalance is puffy swollen hands and feet. Well I can tell you that I have had it in my hands a few times now. You can hardly close your hands. When this happens, its likey that your feet are reacting the same way causing you to go up a shoe size and - OUCH - your shoes no longer fit. There is the cause of a nasty series of blisters right there.
There are lots of ways of talking in electrolytes in endurance events and during training. They are available in endurance drinks like Gatorade. Blisters are my number one fear for the Parish so this is now my plan. I am going to drink Gatorade to try and prevent blisters.
If you had told me that a week ago I would have laughed too :-)