These are clubs, where the membership criteria is that you must have finished "100 Marathons".
The definition of a "Marathon", is generally left to the discretion of the club committee, but in general most clubs agree on the following:
-The distance must be 42.195km/26mile 385yards(No longer/No Shorter).
-It must be in a 'Race', and in general, races are held once a year, have a race director, entry form and publish results.
-The Marathon component of an 'Ironman Triathlon', is generally not accepted as a 'Marathon', nor is the "Marathon Split", of a longer race.