It's possible! To be a stronger runner, you are supposed to keep workouts constantly varied, and to do them for time, right?

Same thing as CrossFit. Introducing, the Farlek workout, runs that vary in terrain, duration and speed. During the run, nothing is predictable so your body has to constantly strive to adjust, which is more taxing on the muscles and cardiovascular system. The post-calorie burn is also greater since the overall workout is more challenging overall.

 

For example, this example run below is constantly varied in terms of time and speed, which is a common theme in CrossFit workouts (in regards to the metabolic conditioning aspect of it). The reason for this run is to be able to do (run) whatever duration and speed you are given, at complete random. It's important to do this so you don't plateau or acclimate to any one workout. This improves your overall running economy, and not to mention, can help you lean out as well.

1 minute light jog (active recovery pace), 2 minute run (normal mile pace), 1 minute light jog, 5 minute run, 2 minute active recovery, 1 minute sprint (400 meter sprint pace), 1 Active recovery, 1 minute sprint (400 meter sprint pace), 1 minute active recovery, 4 minute run, 2 minute light jog, 3 minute tempo (70% Threshold pace), 1 minute active recovery, 3 minute tempo (80% Threshold Pace), 1 minute active recovery, 1 minute sprint (400 meter pace), 5 minute run (normal mile pace).