Crocs were not designed to be a running shoe. Views on the usefulness and aesthetics of Cros are certainly mixed and strongly held. They were originally designed as a shoe for boating and then it became a popular and somewhat maligned fashion item.
However, a number of runners have used Crocs as running shoes and have done alright in them, some several completing marathons running in them.
Having said that, the general advice would not to use them as a running shoe unless you have no other alternative and if you have to use them as a running shoe, then give yourself some time to adapt to running in them before completing a marathon wearing Crocs.