I do a mix of workouts that includes long runs, often with marathon pace mixed in, some tempo runs, but with fartlek type workouts more frequently than interval sessions.
A two week cycle might include two long runs of 17-20 miles (one with 7-10 at marathon pace) and medium on of 14-15 (with 8-12 miles at marathon pace), a modified interval session (e.g., 1 min surges at 5K pace with a brisk recovery--kind of like a 30-40 workout but not as intense), or some repeat miles at 10K pace, or simply a race. I will mix in about one tempo type (LT) run every two weeks. The other days are recovery, 6-10 miles.