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Summary: To plan training for a marathon, it is best to choose the marathon first and schedule a program that progresses up to the marathon date. Create a 12- to 16-week running program with fitness advice from a record-breaking Olympian in this video on running marathons.
"When my coach and I are designing our training for our marathons what we do is we pick a marathon and then we'll work backwards from there as we're planning and what we'll do is we'll usually allow 12 to 16 weeks and we'll break it into 2 week cycles all leading up to the marathon. What we'll do is we'll have the two week training blocks get more and more challenging as they go so maybe for example I might start off with my first tempo run being a 4 mile tempo run at marathon pace and then 2 weeks later I'll repeat the same effort only I might extend it to 6 or 8 miles and it just continues to progress as we go leading up to the marathon."
eHow Article: Training: An Overview