From Wikipedia:
Paris-Brest-Paris (PBP) was originally a 1200km bicycle race from Paris to Brest and back to Paris. It is the oldest bicycling event [beginning in 1891] and is still regularly run.
Since 1951 it has not been a race but a challenge ride with upper and lower time limits. The “First Finishers” get the same medallion as those who scrape inside the 90 hour limit.
This video features footage from the ’83 and ’87 PBP rides. Would love to do this myself in 2011 !