I ran 5 miles along the Jordan River Parkway, and three of the miles were "fartlek miles". During the first mile, I ran my comfortable resting pace to warm up. Then for the next three miles I ran fartleks, and then I ran the last mile at my resting pace to cool down. I felt good during and after the run. Here is a link for those not familar with fartleks.
The fartleks ranged from about 50 feet to maybe 400 feet. In general the shorter fartleks were run at a faster pace than the longer fartleks (the shortest were sprinted), but all of them were run faster than my resting pace. After each fartlek I ran my resting pace to recover from the burst of speed. I ran my resting pace until my body felt ready for another fartlek. One of the advantages of fartleks is that they don't put a lot of stress on your body, due to the short distance that they are run. In other words, you apply the stress of faster running to your body for a relatively short distance, such that your body can easily handle the stress. My kicks at the end of each run are fartleks.
I practiced acceleration-gliding after each fartlek and during the change to my walking breaks.
The temperature during my run was in the mid 30s (F), but we are in an inversion and the air was polluted with haze and automobile smog that can't dissipate into the air. The Salt Lake valley is surrounded on three sides by mountains, and this "bowl" creates inversions in which cold air is on top of warmer air, and the cold air prevents the smog from dissipating. It was so bad today that I couldn't see the Wasatch Mountains that are shown in the pictures of the Parkway in my right side-bar to this blog.