How to prevent sores on feet when walking.?

At the weekend i was training for an expedition of thirtyfive miles, and by the end of it my feet were covered in weepy open sores that were extremely painful. I had put plasters over the sores after the first day of the expedition but they just came off after i got my feet wet. ( my feet being wet couldnt be prevented as we were walking across bog on the moor, and doing river crossings) anyone have any ideas of how i can prevent these sores would be incredibly apreciated, or the best way to treat the ones i currently have would be incredibly apreciated!