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Disadvantages Of Doubles
Interestingly, the same advantages described above can actually be disadvantages if you’re not an experienced runner with a significant aerobic base. Simply speaking, once you’ve been training at a high level for 3-5 years, the aerobic advantages of an easy run start to follow the path of diminishing returns. You still derive benefit from an easy 8-10 mile run, but not as much as a beginner runner whose aerobic endurance isn’t anywhere near its limit.
60-90 Minutes Is Optimal For Building Aerobic Endurance
If you haven’t been running higher mileage (50 miles per week) consistently for at least 3-5 years, the bulk of your improvements are still going to come from improving your aerobic endurance. Therefore, maximizing the number of runs you spend in the 60-90 minute range (widely considered to be the critical time threshold for enhancing aerobic fitness at the cellular level) is the most effective training method. Doubling will lend itself to more runs in the 30-45 minute range, which are still beneficial, but don’t increase endurance as much as a 60-70 minute run will.
Recovery Can Be Hampered
While easy runs do promote recovery by enhancing blood flow, they can also make you tired, as you’ve undoubtedly experienced at one time or another. If your fitness isn’t yet at a level where a 30-40 minute run barely leaves you breaking a sweat, the increased stress of a second run can actually hamper recovery instead of promoting it.
You Feel Like You’re Training All Day
Running doubles requires significant dedication. It’s not easy to lace up the shoes twice per day, even if it’s only a few times per week. While I don’t think this would actually stop any runner from trying them, it can be a burden on your sleep needs, eating habits, and general attitude toward training.
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