Marathons are long distance races on rivers, lakes, estuaria or even open sea. The course may include obstacles such as shallows, rocks and portages. Under ICF rules, the minimum distances for international races are 20 km for men, and 15 km for women.
The races may be divided in several parts and/or several days. World Cup and World Championship races normally are about 35 to 40 km long. As there are no maximum distances, marathon racing has its extremes, such as the Devizes to Westminster Marathon (125 miles, i.e. about 200 km).