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.
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.