my intuition says that interval running is the best regime for this, and most time efficient (like 15 mins per session: 6x30sec sprints + 6x2m jogging)
I believe there are plenty of studies which show that #1 this is most effective with a strong base, and #2 this regime is only effective for 8-12 weeks, at which point in time you stall in progress, and need to go back to base building again.
