Summer is for base building but try to get a rainbow of all the training types each week. Think of it as a way to prepare yourself for the coming faster workouts, as well as building a "base" for those systems as well.
For example: the two things I like to do is have a sprint development day, and a rainbow day in the middle of my easy running weeks.
distance runners need sprint work too, PURE sprint work, preferably on hills. just do half a dozen uphill 60-80 meter sprints after a run. build these up throughout the year to keep some pop in your legs.
for my rainbow day I like to throw in some pace work for different distances, but have them short enough that they are super easy. like on your interval day, just throw a few 400's at 5k pace, 300's at 3k pace, and 200's at mile pace. or whatever distance floats your boat, this would be after a run so just do whatever is comfortable and seems easy. its not a full blown workout.
this way when you do longer intervals you are ready and have already been working up to them.
hope this helps. good luck!