A Centurion is anyone who has walked 100 miles within 24 hours in competition (in Great Britain).

History tells us that though the 19th century the foot servant of a lord often walked, jogged or trotted behind his master's coach.  The practice grew of masters setting their "man" to complete a certain distance in a fixed time for a wager with a fellow aristocrat. 

A most notable feat occurred in 1809 when Captain Barclay, for a wager of 1000 guineas, set out to walk 1000 miles in 1000 hours on Newmarket Heath, near Cambridge in England. 
He completed the walk in 42 days with never much more than one hours rest, losing some 28lbs weight in the process.

Pedestrianism was then of a long distance, endurance nature and usually with money involved.  Still we might imagine that this was the precursor of modern ultra-athletics (without the money!)

1911 --  The Foundation: birth of the Centurions

Between 1902 and the Centurions foundation in 1911 walking as an (amateur) athletic sport had become established (professional athletics still existed)  and some 50 people had qualified in 6 events by the above definition.  At the inaugural meeting, James Edwin Fowler-Dixon was accorded the number "1" for a performance in 1877 and was elected the first president.

The official history notes that "it was decided to form a Brotherhood to be called "Centurions".

The idea of forming the body was conceived by E.R. Bob Gillespie, who had walked 106 miles non stop in a 24 hour race in 1908. The formation meeting was held at the Ship and Turtle, Leadenhall Street, London on 11 May.  Mr James Edward Fowler- Dixon , who had walked 100 miles in a recorded time of 20 hours 36 minutes 8 seconds,  being the senior and longest qualified person present,  was elected President and given the auspicious membership number 1.

The rules for membership were that an applicant must have fulfilled the performance criteria and the application must be unanimously accepted by members at a general meeting. Membership numbers would be awarded in strict sequential order of qualification/ election. (Only 2 numbers have ever not been used, 99 and 453, the former due to a disputed performance, the latter due to the applicant subsequently being found already to hold a number.)
The Centurions remains exclusive in demanding a definitive high standard of performance and having no alternative membership criteria.

 To this day we refer to each other as "brothers", notwithstanding that we have lady members of great distinction.  There are no honorary members, each and every one has attained the magic 100 miles.

There are now several other Centurions "fellowships" - Dutch Centurions, the Continental Centurions of the Netherlands, the Australian Centurions, New Zealand Centurions , US Centurions and lastly Malaysian Centurions. > More information

The site of 131 Leadenhall Street has undergone many changes since those early days.

Taking a walk around the area, this is what we found: Opposite the  Lloyd's building is what we used to call 'Commercial Union Square'  and on the edge of the square (Bishopsgate side) is a modern building, the ground floor level of which is now a large food-only branch of Marks and Spencer. I would think that, as the the numbers jump from 120 to 140, No. 131 was originally on this site - somewhere around the sandwiches/fresh fruit area!!