"In the middle distance world, this concept of losing speed is widespread. Go to any coaching conference, hang around enough coaches, or just talk to your athletes and you’ll hear this concept over and over. It’s used as an argument against doing any kind of aerobic work, regardless of sport or event. It’s thought that only those “slow” distance kids should do long work because speed doesn’t matter much in their world. So they can afford to be slow.
"But here’s the thing, if you are losing speed that much during training, it’s not the long aerobic work that is doing it, it’s that your training sucks."
Read it at http://www.scienceofrunning.com/2015/04/the-myth-of-losing-speed.html
"...you need a blend of aerobic and pure speed work throughout the year."
Yep. Middle distance runners can do large amounts of endurance training. And with speed work all year round, properly integrated into their training program, won't lose speed.