If you don't, you may want to. It's a training method.........seriously.
"Fartlek" is a Swedish term for "speed play". Essentially it's based on the idea of implementing short bursts of speed into your workout. For instance if you're jogging for 45 minutes, you would implement 5 to 10 short intervals where you sprint at 90-100% effort. Or when walking, you would add short bursts of jogging or even sprinting.
It's not quite as demanding as traditional interval training because it allows you to implement the intervals according to your body. Personally, I would need some sort of metrics for this such as a minimum number of intervals to complete, even though I may execute them when my body is ready throughout a particular session. That's just me though.
A slight variety of this method is something I used to employ. I had both an elliptical machine and stationary bike at home. I would typically warm up for 7 or 8 minutes then...Fartlek. Jump on the bike for 60 seconds and peddle away as fast as I possible could. Then back to the elliptical, jumping back to the stationary bike every 4 to 7 minutes for the intense 60 second burst. It intensifies the fat burning quotient of your workout, even if you start at 30 second bursts.
At the time, I had no idea it was called the Fartlek method. So give it a try and let me know how it works for you! Happy Fartleking!