It takes about two minutes to elevate VO2 to capacity levels. So if you are going to train for capacity the real work begins when the work interval gets beyond two minutes. Unless the work is so short that the recovery is incomplete, in which case VO2 remains elevated throughout the workout.
Complete 3 min intervals at your 2 km pace + 10 sec.
Begin with 6 intervals.
Include this workout once a week and add 1-2 intervals per session until you reach 12.