Running fartleks is the most basic and unstructured of interval training, and I recommend this type of speed workout for those just beginning to dabble in higher intensity runs, or just starting to start speed work up again at the beginning of a season.
In basic, a farlek run consists of a normal, comfortable pace run with intermittent surges throughout. These surges can be as planned or unplanned as you desire. For examples, I’d start out with at least a mile to a mile and a half of an easy run just to get warmed up. Then every so often (every half mile, every 3 minutes, every time you feel fully recovered from the last one, every time your heart rate drops back below a certain point, etc) just pick it up. I definitely don’t recommend an all-out sprint, because you want to be able to keep going, but the point is to get your heart rate up and your breathing more elevated than normal. Start by doing this 5 or 6 times throughout your run and then finish with another easy mile or so cool down of easy running.
On days when I didn’t want to be a slave to my watch and mileage, I’ll base the whole workout on counting steps. After the warmup phase, I’ll do something like 50 steps fast, 200 steps easy, and repeat this until I’m done.
The key is to push yourself and get accustomed to increased heart and breath rates and start to get more used to being a little uncomfortable. It’ll get easier and allow you to increase the number of repetitions, the distance of reps, and/or the speed of the reps.