You're trying to lay the foundation to withstand the racing schedule at a point in the season in which the intensit of workouts rises as well. You have to pay the aerobic price while not getting too far away from basic leg speed. The speed can be maintained without compromising the aerobic developement. Hills, fartlek, and tempo can be added judiciously throughout the summer, but the bulk of a summer training program should be steady state runs. Put the most emphasis on increasing volume safely by not adding too much intensity too soon. Once every 10-14 days, you can do what I call a "plateau buster" of some sort-usually a tempo run or time trial just to shock the system and confirm that you are, in fact, gaining fitness.
Consistency is the key. Be consistent in terms of your training, sleep, nutrition etc. and keep a detailed journal of your training so you can look back to see where your gains were made.