How long is a marathon?
The standard marathon distance is 42.195 km (26 miles 385 yds) although it wasn't always that distance.
The name "marathon" comes from the legend of Pheidippides, a Greek soldier who, according to legend, was sent from the town of Marathon to Athens to announce that the Persians had been miraculously defeated in the Battle of Marathon. It is said that he ran the entire distance without stopping, but moments after proclaiming his message to the city, he collapsed dead tired. There is no evidence that any such event took place; according to the Greek historian Herodotus, Pheidippides was a messenger who ran from Athens to Sparta. The legend that he ran from Marathon to Athens was invented by later writers and appears in Plutarch's On the Glory of Athens in the 1st century AD. The International Olympic Committee estimates the actual distance from the battlefield of Marathon to Athens is about 34.5 km (21.4 miles).
The idea of organizing the race came from Michel Bréal, who wanted to put the event on the program of the 1896 Summer Olympics in 1896 in Athens. This idea was heavily supported by Pierre de Coubertin, the founder of the modern Olympics, as well as the Greeks. The Greeks staged a selection race for the Olympic marathon, and this first marathon was won by Charilaos Vasilakos in 3 hours and 18 minutes. Spiridon "Spiros" Louis, a Greek shepherd, finished fifth in this race but won at the Olympics in 2 hours, 58 minutes and 50 seconds, despite stopping on the way for a glass of wine from his uncle waiting near the village of Chalandri. The women's marathon was introduced at the 1984 Summer Olympics Los Angeles, USA.
World Records and “World's Best”
World records were not officially recognised by the IAAF until 1 January 2004; previously, the best times for the Marathon were referred to as the 'world best'. Courses must conform to IAAF standards for a record to be recognized. However, marathon routes still vary greatly in elevation, course, and surface, making exact comparisons impossible. Typically, the fastest times are set over relatively flat courses near sea level, during good weather conditions and with the assistance of pacesetters.
For most runners, the marathon is the longest run they have ever attempted. Many coaches believe that the most important element in marathon training is the long run. Usually recreational runners try to reach a maximum of about 20 miles (32.2 km) in their longest weekly run and about 40 miles (64.4 km) a week in total when training for the marathon. More experienced marathoners may run a longer distance, and more miles or kilometres during the week. Greater weekly training mileages can offer greater results in terms of distance and endurance, but also carries a greater risk of training injury. Most male elite marathon runners will have weekly mileages of over 100 miles (160.9 km).
Many training programs last a minimum of five or six months, with a gradual increase (every two weeks) in the distance run and a little decrease (1 week) for recovery. For beginners looking to merely finish a marathon, a minimum of 4 months of running 4 days a week is recommended (Whitsett et al. 1998). Many trainers, including Dr. Daniels, recommend a weekly increase in mileage of no more than 10%. It is also often advised to maintain a consistent running program for six weeks or so before beginning a marathon training program to allow the body to adapt to the new stresses. 
During marathon training, adequate recovery time is important. If fatigue or pain is felt, it is recommended to take a break for a couple of days or more to let the body heal. Overtraining is a condition that results from not getting enough rest to allow the body to recover from difficult training. It can actually result in a lower endurance and speed and place a runner at a greater risk of injury.
The world record time for men over the distance is 2 hours 4 minutes and 55 seconds, set in the Berlin Marathon by Paul Tergat of Kenya on September 28, 2003 (ratified as the world record by the IAAF on 1st January 2004), an improvement of 20 minutes and 44 seconds since 1947. The world record for women was set by Paula Radcliffe of United Kingdom in the London Marathon on 13 April 2003, in 2 hours 15 minutes and 25 seconds. This time was set using male pacesetters - the fastest time by a woman without using a male pacesetter ('woman-only') was also set by Paula Radcliffe, again during the London Marathon, with a time of 2 hours 17 minutes and 42 seconds set on 17 April 2005.