as the title suggests. you (or your athlete, to prevent personal biases) can only do one type of workout for the rest of your career. what is it and why. no absurd combinations of workouts where you do a MP workout, VO2max, 800 speed, and hill work all at the same time.
my opinion:
5-7 mile threshold run at 90% of max HR, maybe 20-30 seconds slower than current 5k pace. with warm up and cooldown, you can make it a 13-15 mile day, so its a realistic non-long-run-type workout for the marathon. its obviously beneficial for a 10k-half marathon event since it is in the same pace range. It is the hardest aerobic stimulus that can be maintained for a long time without rest and without feeling like a race, so it is the most comprehensive way to develop a stronger aerobic respiratory system, which benefits every event from probably the 800 up.