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I would like to know if when training in a day with a speed series like 8x400 is more heavy than a 18k longrun. Because yesterday I did my first speed series training and I was sleeping all the afternoon. But when I do my long run of 18-20k my body is ok.

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I think it depends on the effort you give. If you try and run the long distance in your best time, you may feel the same. – Freakyuser Jun 14 '13 at 11:38
up vote 3 down vote accepted

Unfamiliar exercises are harder than familiar exercises. The 8x400 zonked you because you're used to 20k runs. A sprinter used to 8x400 would be zonked after a 20k run.

Things we're not good at or that we don't do are more exhausting.

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