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It's the foundation (cake) you really need to worry about. If the rest of your training is at an appropriate volume, the long run is really just the icing on the cake. I don't know any runners who go longer than 2:30. If you're putting in solid mileage the rest of the week, a 2 hr long run every week (some weeks slow, some weeks fast, some weeks progression) is as long as you need to go.
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So your typical distance runner may be hitting 15-20 miles on their long run. Conventional wisdom has the long run usually equal to 20-25% of the weekly mileage. This is pretty contradictory to standard distance running training. So basically you do some running during the week, then a crazy ass long run on the weekend that probably takes a beginner runner up to 3+ hours to do. A lot of the 'beginner' marathon plans are designed to just get the athlete to the finish.
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