In HS and College I did almost no interval work during the summer. I did do high aerobic runs of 5-10mi 2x a week. I did strides 3-4 times per week. Each year of HS, my 2mi track time became my 2mi Cross Country split the next fall after that summer work. Each year of College, my 5mi Cross Country times dropped by 30-45sec.
Cycled training is critical to getting the body to perform. Just like during a week you do hard days and easy days, you need hard weeks and easy weeks and you need hard seasons / racing and easier base phase. The body can recover from easy to moderate running much easier than hard running. But moderate running is the key. Easy all the time does not stress the body and therefore gives minimal gains.