It seems to have become more fashionable for people wanting to get into shape to run a marathon. I ran a marathon once, and there is a certain sense of accomplishment to it. But relatively speaking, it is a very poor method of trying to get into shape.
Being able to cover the marathon distance, 26.2 miles, is not really a indicator of health. The idea of aerobic training is to build your aerobic capacity or VO2max, not to be able to run endless miles. VO2max usually goes up when people get faster over a certain distance, not necessarily when they are able to cover longer distances. Said another way, a person who can run one mile fast is in much better aerobic condition than someone who can only run a marathon slow.
Improving aerobic condition does not mean putting in tons of miles on the road or bike. A well-balanced program of easy, moderate, and intense training is right way to go.