How many km in a marathon?

How long it is really?
An official Olympic Marathon running distance is exactly 42.195 kilometers.

Facts
Only four men and eight women were able to follow one marathon world record with
another. James Peters set four marathon records between 1952 and 1954, while
Abebe Bikila, Derek Clayton, and Khalid Khannouchi each set two world records in the
race. On the women's side, Greta Weitz set four world records in a row from 1978 to
1983 (the last stood only for one day!), while Chantal Langlace, Jacqueline Hansen,
Christa Vahlensieck, Joyce Smith, Tegla Loroupe and most recently Paula Radcliffe
each broke the marathon record twice.

Perhaps the most famous of all of the world records were the races of Abebe Bikila,
the barefoot Ethiopian, who set world records four years apart while winning the
Olympic Marathons in 1960 (barefoot) and 1964 (wearing shoes).