In the answer to a question on the validity or otherwise of a world record, the answer quotes the rule book which says:
"The start and finish points of a course, measured along a theoretical straight line between them, shall not be further apart than 50% of the race distance." [i.e., 13.1 miles (21.1 km) for the ~26.2 mile (~42.2 km) marathon distance]
Why should this be so? I can understand stipulations about ascent or descent between the start and end, but why should the endpoints need to be close together? (It means that the original 26.2 mile course, from Windsor to White City in west London, wouldn't be legal.)