Fartlek, that is.
Also known as speed work. Or sprints if you are on a track.
And yes. I really enjoy Fartleks. It gives me a chance to improve my pace, as well as get a kick-ass workout. While I do prefer to run outside, I generally like to do my speed work on a treadmill, so as to control my pace, and have some clue as to how fast I am going.
Outdoor speed work is easy and fun. If you are on a track, just sprint about 10 yards as fast as you can, then walk back to the starting point and repeat. If you are out on a regular run, simply pick a landmark, tree, house, mailbox or what have you, and run to that point faster than your current speed. Repeat this throughout your run.
It’s important to do speed work when training for an event to help decrease your per mile pace, but not a necessary part of training if increasing your speed is not a goal for you. It also helps to strengthen your leg muscles, and your core. Not to mention the HIIT is a great way to whittle your middle as well.
Try working in some Fartleks on your next training run!