Can I improve running stamina by increasing intensity rather than duration?
Yes, increasing intensity rather than duration is a perfectly valid way of training, and is even better under the right circumstances.
There are many factors that determine how much endurance you have. Two of the most important factors are the rate at which the athlete's body can consume oxygen (VO2 max) and what intensity of exercise at which lactate begins to accumulate in blood at a faster rate than it can be removed (lactate threshold).
In this 2007 study they recruited 40 nonsmoking, moderately trained male subjects and assigned each of them to one of four groups.
- Long Slow Distance: 70% maximal heart rate
- Lactate Threshold: 85% maximal heart rate
- 15/15 Interval Running: 15 seconds of running at 90-95% maximal heart rate
- 4x4 Minutes of Interval Running: 4 min of running at 90-95% maximal heart rate followed by 3 min of active resting at 70% maximal heart rate
Shown below demonstrates what the heart rates would look like in each group.
They found that both of the higher intensity groups significantly increased their V02 max, yet the lower intensity groups had no statistically significant changes in their V02 maxes. The velocity at lactate threshold was significantly improved by an average of 9.6% for all groups as a consequence of changes in running economy and V02 max. Due to how difficult it is to do the 15x15 at a heart rate of 90-95% HR max, they advised to do 4x4 min training if you want to improve V02 max.